The effect of water activity (aw, 0·995–0·85) and temperature (15, 25°C) on the in vitro inter- and intra-specific interactions between thirteen fungi commonly isolated from maize grain were investigated. The fungi were paired and their interactions given a numerical score to obtain an Index of Dominance (ID) for each species. Aspergillus niger had the highest overall ID in most of the conditions tested, while the Aspergillus species tested were also quite dominant. Fusarium species appeared to be dominant only at high water availability (0·995 aw), while Eurotium species dominated at the lower aws (0·85–0·90). The relative growth rates of each fungus were also calculated under the same range of environmental conditions. In general, Aspergillus, Fusarium and Trichoderma species grew most rapidly under the combination of aw and temperature conditions in which they were able to grow, while Penicillium species had the slowest growth rates. There was a positive correlation between growth rate and ID for Trichoderma viride and three Fusarium spp. but not for Aspergillus and Penicillium and Eurotium spp. The different possible strategies in fungal competition for grain are discussed in relation to these in vitro findings.