Twenty-one species (23 isolates) of marine diatoms were examined for their capacity to utilize analogs of cyanocobalamin for growth at the ecologically significant concentration of 4 ng 1-(1) . Yields due to the analogs were compared to those produced by B12 . Responses of the various clones to the analogs were not all-or-none, but varied continuously; thus, assigning the clones to the conventional B12 specificity types is a convenient but arbitrary classification. The use of 10 and 1% levels of response is suggested for such classification. At the 10% level of response, 11 clones had coliform, 4 lactobacillus, and 8 mammalian specificity patterns. At the 1% response level, 14 had coliform, 5 lactobacillus, and 4 mammalian specificities. All clones exceed the 10% response level on all benzimi-dazole-containine analogs tested. Few clones showed definite enough patterns of response to make them potentially useful for differential bioassay. The clones suggested are clone 675-D (Bidclulphia sp.?), clone F(;) -3 (Fragilaria sp.?), and the estuarine clone of Cyclotella nana (3H).