FRANKFURT, West Germany (AP) _ A poisonous chemical spill in the Rhine has killed half a million fish, deprived towns of water supplies and stirred fears of long-term damage to one Europe's busiest and most scenic waterways.

French officials plan decrees against catching Rhine River fish for the next six months becauase of the spill Nov. 1 during an industrial fire in Basel, Switzerland.

Some critics are calling the spill Chernobale, after the Chernobyl nuclear disaster last April in Soviet Union. Bale is the French name for Basel.

''We don't know how damaged the Rhine is, but there's no doubt that there has been serious harm done,'' Walter Wallmann, West Germany's environment minister, said Monday.

In Brussels, the European Common Market's environment commissioner requested an urgent meeting of government ministers later this month to evaluate the consequences and ''deal with the immediate crisis.''

The Swiss government said member nations of the International Rhine River Commission would hold a special meeting Wednesday in Zurich.

West German, French and Dutch officials accused Swiss authorities of slow reaction and poor cross-border coordination after the accident.

About 30 tons of toxic herbicides, pesticides and mercury poured into the Rhine along with water used to put out a fire at the Sandoz chemical plant outside Basel.

The spill, about 25 miles long, drifted slowly downstream, causing huge fish kills and contaminating water supplies of river towns.

Concern has been intensified by the Rhine's importance as an artery of European commerce and tourist attraction.

The 820-mile-long river rises in a Swiss glacier and passes through or forms borders of Liechtenstein, Austria, West Germany, France and the Netherlands on its way to the North Sea.

French authorities said they had prevented the chemicals from entering tributaries and underground water sources by closing lock gates into canals and secondary waterways.

Unkel and Bad Honningen in West Germany's northern Rhineland indefinitely closed off their normal water supplies, which are a combination of Rhine and ground sources.

''They will continue to be served with water by a 24-hour firetruck service until further notice,'' said Detlef Diehl, spokesman for Environment Minister Wallmann. He said other Rhine communities either customarily used only ground water or had switched to it.

Precuationary measures also include cancellation of some Rhine cruises, which are important to the West German tourist industry.

The chemicals entered the Netherlands over the weekend and neared the Rhine delta on the North Sea late Monday, Dutch environmental authorities reported.

A second patch of mercury-tainted water from a container leak at the Sandoz plant that was related to the fire made its way down the river Monday in West Germany. Company spokesman Benedikt Hurni said toxic concentrations were ''minimal'' compared with the first spill.

Bruno Voskuil, spokesman for the Dutch Environment Ministry, said the toxins spilled initially were diluted by passage through West Germany and France and killed only some aquatic micro-organisms in the Netherlands.

He said officials used Holland's system of locks and movable barriers to push the pollution into the sea as swiftly as possible and stop it from spreading into other waters.

At a news conference Monday in Bonn, Wallmann said the chemical spill had killed 500,000 fish. He said he would meet with chemical industry leaders Tuesday to discuss tougher safety measures.

Swiss insurance investigators found that some Sandoz chemical storage practices had not met minimum safety standards, he said.

''We are not yet able to name all the chemicals that may be in the Rhine as a result of this accident. There could be more,'' Wallmann said. It is too early to evaluate the long-range ecological impact, he added.

He said authorities in Switzerland underestimated the accident and waited too long before warning neighboring countries.

A Sandoz spokesman, speaking on condition of anonymity, denied Monday that the company had violated storage regulations but acknowledged that some safety measures may have been inadequate.

The government of one West German state on the Rhine, Baden-Wuerttemberg, announced Monday it would seek reparations from the Swiss company.

Claudia Roth of West Germany's Greens party summed up the environmental concern: ''How are you going to put a price on the Rhine itself or wildlife preserves in the tributaries that become wastelands? We're going to make sure this won't happen here again.''