From November 1990 to November 1992, immature mosquitoes were sampled from the shoreline and from emergent beds of the submerged aquatic plant Hydrilla verticillata at the Ross River reservoir, northern Australia. Aerial mapping of Hydrilla beds was done in conjunction with sampling to estimate total immature mosquito numbers. Larvae of 7 species were found. Culex annulirostris. Anopheles annulipes s.l., and Anopheles amictus comprised 80.4% of the total. Peak larval densities occurred in the late wet season period in both habitat types (March to May) but Hydrilla generally supported higher densities, particularly of An. annulipes s.l. (43.7% of the total sample), than the shoreline habitats. Anopheles annulipes replaced Cx. annulirostris as the predominant taxon when 1990-92 data were compared with data for 1985-86. The Hydrilla beds supported on the order of 5.6 x 10(9) immatures during the period of peak density. This suggests that where human exposure is of concern, mosquito control in habitats such as Hydrilla is warranted.