We describe activity patterns, diet, reproduction, sexual dimorphism, and thermal ecology of the semiaquatic teiids Crocodilurus amazonicus and Dracaena guianensis, from two localities in the Brazilian Amazon. Most C. amazonicus were first sighted in water or on open ground, were active during the hottest hours of the day, and usually had low body temperatures associated with substrate temperatures. Dracaena guianensis were found mainly perching on shrubs and used higher perches located closer to the center of lakes compared to C. amazonicus. Both species appear to rely primarily on crypsis to escape detection by predators but will dive into water as a final means of escape. Crocodilurus amazonicus has a broad diet which includes terrestrial and aquatic prey, particularly spiders and hemipterans, whereas D. guianensis feeds primarily on aquatic snails. No association between body dimensions and prey dimensions was evident. Sexual size dimorphism was not significant in either species, contrary to results reported for other teiids elsewhere, but males of C. amazonicus had relatively longer bodies and tails than females. Clutch size of both species was small relative to their body size and relative to other Amazon teiids, apparently influenced by their semiaquatic habits and by locomotor constraints. We found reproductive females during March (wet season) and July (dry season), suggesting an extended reproductive season.