SUMMARY: Streptococcus sanguis, S. bovis, S. mutans and S. salivarius, and also Escherichia coli for comparison, were grown separately in a chemostat under ammonia- and glucose-limitation. Bacterial extracts were assayed for ammonia-assimilating enzymes. For E. coli the level of glutamate dehydrogenase decreased and the levels of glutamine synthetase and glutamine (amide): 2-oxoglutarate aminotransferase (glutamate synthase) increased following a change from glucose- to ammonia-limitation. In contrast, ammonia-limited streptococci contained much higher levels of glutamate dehydrogenase than glucose-limited streptococci, and although a glutamine synthetase was detected in S. bovis, glutamate synthase could not be detected in any of the streptococci. Low activity of a NAD-linked alanine dehydrogenase was found in S. bovis and S. sanguis. These results suggest that the only pathway of ammonia assimilation in streptococci involves glutamate dehydrogenase, irrespective of the nature of the growth limitation. Glutamate dehydrogenase extracted from the different species of Streptococcus had similar Michaelis constants and optimum pH values.