The population dynamics of Anopheles pseudopunctipennis larvae were studied in a foothill region near Tapachula, Mexico. Systematic surveillance of wet-season and dry-season habitats was conducted during 1990 and 1991. Sampling along transects of the Coatan River was employed to quantify habitat availability and population densities of larvae during the dry season. During the wet season, larvae were most abundant in temporary habitats, such as seepage springs, rain pools, and pools in stream and river margins. The temporary habitats disappeared during the dry season, which occurred concurrent with increasing densities of larvae in dry-season habitats within transects along the Coatan River. The great abundance of the dry-season riverine habitats, viz., small pools with filamentous algae, resulted in peak densities of host-seeking adult populations in villages associated with the river. During both seasons, there were significant associations between the presence and abundance of larvae and habitats containing filamentous algae, and secondarily with selected aquatic and semiaquatic plants. There was a significant correlation between mean numbers of larvae per habitat and mean numbers of breeding sites in the transects. Overall, An. pseudopunctipennis larvae were very abundant during the dry season and relatively uncommon during the wet season.