PARIS (AP) _ In the fashionable French trench, tacky khaki is out and polyester is in.

The berets can stay but unpleated pants got their marching orders as the army paraded its new uniforms Tuesday at an unusual fashion show.

Not everyone was wild about the styles; some full-figured soldiers complain they may have to wear girdles to squeeze into the new skirt-pants for women.

The parade showed blousons for all enlisted persons and straight, pleated pants for men and women. The lighter fabrics and colors with contemporary style and practicality make army gear look made more for fashion than fighting.

''It was time for a change,'' said Gen. Gilbert Forray, chief of staff of the army, who helped engineer the creation of the uniforms, which had not changed for 40 years. The uniforms were ordered from the house of Balmain.

Balmain, who also recently redesigned uniforms for the French police, was chosen after polls conducted within the army to determine which cuts, colors and fabrics would best suit the needs of today's army.

The clothes were designed by Balmain's couture designer Erik Mortensen and Patrick Aubert, head of the company's masculine fashion department.

The fashion parade, after opening with some flamboyant red-white-blue braid-edged redingotes from the time of Louis XV, displayed what the new officers, gentlemen and women will wear in the service beginning in 1991.

The new blousons with a shoulder-to-waist pleat are both sober and chic. The clothes are topped with kepis for officers or sharp berets with wheel- shaped pins.

''And thank heaven they didn't get rid of the kepi,'' said one male fashion observer, who noticed that male officers would still sport the hat silhouette as distinctive as Charles de Gaulle's.

Khaki is out in the new military palette, a lighter shade of putty or pewter called ''terre de France'' is in, and in a lighter fabric - a mix of wool and polyester, rather than heavy twill.

''It's more practical, a lot less gear is involved now, and it's more elegant,'' said Capt. Denis Brunel.

Except for navy blue uniforms for the infantry, the army will wear the new greige, and women will no longer wear their slate blue, but the same putty color as men.

''That's good, and makes women more the equal of men,'' said Sgt. Patrick Gall. ''My wife is in the army too, she'll surely approve.''

A layered look called ''modular'' means officers and gentlemen can go without their coats even when confronting superiors. The light cotton poplin shirts underneath have epaulets, tab pockets and may be substituted for a jacket in warm weather.

''I think the uniforms look very good and easy to wear,'' said Col. Abraham Vigilant of the artillery corps. ''They give an impression of serenity, which we need in the army. I'm a philosopher - with my feet on the ground.''

The only sticky problem, so to speak, is in women's skirt-pants, a new knee-length pair of bermuda shorts with a wrapped skirt covering.

''What about real women? I'm waiting to see how the new uniforms will work out on us,'' said Sgt. Chantal Grosjean, a full-figured soldier about 5-foot- 4. ''A real woman has thighs and a stomach. What happens with these uniforms? We might have to wear girdles.''

And she also thought they might prove difficult in summer. ''We have to sit around on those fake leather chairs all day long.''