Observations were made in July and September of 1982 and in January of 1983 in Polk County, Florida, to determine differences in the relative abundance of Mansonia dyari and Ma. Titillans larvae on water lettuce plants during day and night. The results showed that during certain times of the year the number of larvae attached to plants during the day was significantly higher than those attached at night. During July and September only Ma. dyari was collected, but during January, of 3541 larvae collected 89.5% were Ma. dyari and 10.5% were Ma. titillans. Members of both species appeared to detach at night in equal proportions. Although large numbers of larvae were detached at night, this study did not reveal their specific location during detachment.