We studied the seasonal patterns of 3 community attributes (breeding site index, richness, and diversity) of groundwater mosquito immatures and their associations with meso- and microhabitat factors in the Paraná Lower Delta, Argentina, from December 2009 to November 2010. Monthly collections at 4 sites yielded 2,313 mosquito immatures, belonging to 19 species assigned to 6 genera. Immatures developed in a wide range of microenvironmental conditions (water temperature 4.0–30.2°C, pH 5.4–9.1, electrical conductivity 0.02–6.33 mS/cm, and dissolved oxygen 0–235.6%), and their occurrence was significantly associated with lower water temperature and dissolved oxygen values. Most frequently captured species were Culex dolosus s.l. (58.6%), Aedes crinifer (22.9%), and Cx. intrincatus (19.0%). Breeding site index was highest in winter (0.76) and lowest in spring (0.54). Species richness and diversity varied significantly among seasons (peaking in summer), land uses, and habitat types.