Persistence of 1.12, 2.24, and 3.36 kg/ha of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] applied preemergence to corn (Zea mays L. ‘Funk's 4761′) was determined in three soils, each providing a range of pH from 5 to 7. Samples of surface soil from each plot were collected at intervals after atrazine application until there was no evidence of phytotoxicity to oat (Avena sativa L. ‘Florida 501′) in bioassay. Persistence (Y50) was defined as the number of days following atrazine application until bioassay indicated 50% relative growth of oats (no atrazine = 100% growth). This period ranged from 24 to 178 days, depending upon soil type, pH, and atrazine rate. Persistence of atrazine increased with increasing soil pH. The effect of soil pH was essentially the same at each atrazine rate and did not vary appreciably with years in a given soil. In McLaurin sandy loam, atrazine persisted 8 to 9 days longer with each unit increase in soil pH. In Hartsells fine sandy loam and Decatur silt loam, atrazine persistence increased 9 to 13 days and 29 days, respectively, with each unit pH.