A.M. Turing, in a 1941 personal communication to 1.1. Good, suggested a formula for estimating probabilities of words in text and, more generally, of species in a mixed population of various species. The formula is particularly interesting in a “small sample” situation where events of interest did not occur at all. It is remarkable that Turing’s formula can be obtained by significantly different statistical methods. © 1991 by American Sciences Press, Inc.