Colonization of barley grain byFusarium sporotrichioidesand T‐2 toxin formation in the presence ofAspergillus flavus, Penicillium verrucosumandHyphopichia burtoniiwere studied at 20 and 30°C and at 0.97, 0.95 or 0.90awduring 3 weeks’ incubation. Colonization of grain was assessed from frequency of seed infection and numbers of colony forming units (cfu) produced and by observation of hyphal extension on the grain surface from germinating spores while concentrations of T‐2 toxin were determined by enzyme‐linked immunosorbent assay using monoclonal antibodies. Germination ofF. sporotrichioidesspores was unaffected by the presence of other species under all conditions. However, subsequent colonization of barley grain byF. sporotrichioideswas either completely inhibited or markedly decreased by the presence of other fungi irrespective of theaw, temperature or competing species involved. T‐2 toxin production occurred only at 20°C and 0.97 or 0.95aw, and mostly within the first 7 days: production of T‐2 toxin byF. sporotrichioideswas significantly greater in the presence ofA. flavusandP. verrucosumthroughout the 3‐week incubation period under most conditions. T‐2 toxin production was only slightly decreased by the presence ofH. burtoniidespite the apparent lack of growth ofF. sporotrichioides.