New York City seems a fine place for an observant Jew to keep kosher. There are specialty shops for the ultra-Orthodox and for those of less strict beliefs.

And it all works out just fine, provided you don't get thirsty. Some rabbis now say that New York City tap water -- for a century a gold standard for cleanliness -- is not kosher.

These rabbis have recently discovered that there are tiny creatures, called copepods, in the unfiltered water that streams into the city from upstate. These tiny organisms are harmless. But they are crustaceans. And crustaceans are not considered kosher.

Over the past two weeks, concern about the copepods in the water has grown into a matter of intense debate among the city's Orthodox Jews.