This study, conducted in 3 parishes of Louisiana, documented the distribution pattern of Anopheles quadrimaculatus and An. crucians during the first crop season. Ratios of numbers of 3rd and 4th instar larvae of An. quadrimaculatus to the number of An. crucians 3rd and 4th instar larvae (Q/C), varied both spatially and temporally. The mean Q/C ratio during 1984 was 1.37; during 1985 it was 1.40. The ratios changed from week to week during each year, and increases or decreases in a particular week of 1984 were paralleled by increases or decreases during that same week of 1985. The changes in relative abundance of the 2 species were strongly related to the timing of irrigation practices and to a succession of habitat conditions. Anopheles quadrimaculatus larvae were dominant or exclusively present for several days after fresh water was added to the field. Anopheles crucians larvae then became dominant or exclusively present as the habitat progressed towards a eutrophic condition. Irrigation influenced the conditions of the habitat by freshening and deepening the water, causing a reversion towards a more oligotrophic condition.