SAN FRANCISCO (AP) _ A hundred years have passed since Phin's filler ran that day,

It stirred the souls of millions but was worth just five bucks pay,

Ol' Casey fanned with two men on and caused Mudville to pout,

But somewhere hearts are lighter because Thayer cranked it out.

Often imitated but never matched, ''Casey at the Bat'' hit the century mark Friday with its place in history still secure as one of America's best-known comic verses. The mighty slugger's whiff remains perhaps the most famous strikeout of all, outdoing any involving Reggie Jackson or Carl Hubbell.

The San Francisco Examiner first published the ballad June 3, 1888, placing it inconspicuously on page 4 between an editorial on the Republican national convention that nominated Benjamin Harrison and a cynical column by Ambrose Bierce.

As the newspaper noted in reprinting the ode Friday, its impact wouldn't have been nearly as great had the mighty Casey homered to win the game for the Mudville nine that day. And if he'd hit a two-out single to drive in Flynn, who was ''a-hugging third,'' and Jimmy Blake (''safe at second'') for a 4-4, ninth-inning tie, he'd be just another fictional ballplayer.

''The story of Casey has become an American myth because Casey is the incomparable, towering symbol of the great and glorious poop-out,'' wrote Martin Gardner, a former newspaper reporter who studied the poem's factual basis and compiled a collection of ballads about Casey.

The poet himself was a bit of a Casey.

Born in Lawrence, Mass., Ernest Lawrence Thayer was a brilliant philsophy student at Harvard University, where he developed close friendships with philosophers William James and George Santayana. He edited the Harvard Lampoon and faithfully attended every Crimson baseball game in 1885, when he graduated with highest honors.

He accepted an offer from Harvard chum William Randolph Hearst to write a humor column for the Sunday supplement in Hearst's Examiner. Two years later, when he was 24, ''Casey'' ran as his final contribution - as usual under the byline of Phin, his nickname.

A few Eastern papers reprinted it, but the ballad only caught on after capturing the fancy of comedian William De Wolf Hopper.

A bored Hopper tried reciting it one night in May 1889 during a comic opera on Broadway. The audience was so delighted that, by Hopper's count, there were more than 10,000 times in his career when he cleared his throat and began: ''The outlook wasn't brilliant for the Mudville nine that day ... ''

After disgustedly watching several people falsely claim authorship of ''Casey,'' Thayer spent his life immersed in great books and philosophy, and admitted frustration shortly before his death in 1940 that he'd never succeeded in writing anything serious.

Not unlike his slugger, Thayer was chagrined at how he'd left his mark. He called the ballad ''nonsense'' and huffed that ''its persistent vogue is simply unaccountable.''

Balderdash, say countless fans.

A group of seventh-graders in San Francisco brought the tradition alive for a new generation this week, acting out the poem for the entire student body. Downtown, a toddler gazed up in awe at a huge papier-mache Casey, bat firmly in hands, erected as a tribute to the poem's anniversary.

Gardner said it's easy to understand the ballad's popularity.

''It is not great poetry,'' the ''Casey'' scholar wrote. ''It was written carelessly. Parts of it are certainly doggerel.

''Yet it is almost impossible to read it several times without memorizing whole chunks, and there are lines so perfectly expressed, given the poem's intent, that one cannot imagine a word changed for the better.''