Incorporation of [methyl-3H]thymidine was measured in solar saltern ponds of different salinities. Estimated doubling times of the bacterial communities were in the range of 1.1 to 22.6 days. Even at the highest salt concentrations (NaCl saturation), relatively rapid thymidine incorporation was observed. In an attempt to differentiate between activity of halophilic archaeobacteria (the Halobacterium group) and halophilic eubacteria, taurocholate, which causes lysis of the halobacteria without affecting eubacteria, was used. At salt concentrations exceeding 250 g/liter all thymidine incorporation activity could be attributed to halobacteria. Aphidicolin, a potent inhibitor of DNA synthesis in halobacteria, completely abolished thymidine incorporation at the highest salinities, but also caused significant inhibition at salinities at which halobacteria are expected to be absent. Attempts to use nalidixic acid to selectively inhibit DNA synthesis by the eubacterial communities were unsuccessful.