Variability in growth rate was observed among S strain isolates of Aspergillus flavus from western Arizona. Isolates were grouped into fast and slow growing biotypes and compared for radial growth, and sclerotia, spore, aflatoxin, and pectinase production. Slow and fast biotypes were distributed equally among three collection areas with 35% of 300 S strain isolates classified as the slow biotype. Fast biotype isolates tended to produce more conidia than slow biotype isolates. However, both types produced similar quantities of aflatoxin B₁ in vitro (>200,000 ng/g). Plate assays indicated that both producers and non-producers of pectinase P2c were distributed among both biotypes. The results document divergence within the S strain isolates of A. flavus.