Tadpoles are inhibited in growth when raised in water previously crowded by other larger tadpoles. The 'alga-like' cells described in earlier crowding studies were found in the fecal material and intestinal tracts of inhibited tadpoles. Results of 38 crowding experiments using 17 species of tadpoles show that the effects of the inhibitory cell are nonspecific. Water (with cell-laden fecal matter) from crowded tadpoles of one suborder can inhibit the growth of assay tadpoles of a different suborder. There is no decreasing level of inhibition with phylogenetic distance. Bufo woodhouse is the only one of 17 anuran species tested that seems immune to these inhibitory effects. Tests with Bufo woodhouse indicated the existence of a possible intraspecific growth-promoting substance. The substance may also inhibit some species, and not affect others. Metamorphosing tadpoles may release a nonspecific growth-stimulating substance (thyroxine?) into the water, which may alleviate the effect of inhibitory substances.