The most important fungi consistently isolated from tomato plants naturally infected by the basal stem rot disease in Northern Nigeria were:Fusarium oxysporum, F. equlseti, F. solani, F. fusaioides, F. acuminatum, Trichoderma viride, T. harzianum, T. hamatum, Aspergillus flavus, A. niger, A. sydowiiandA. foetidus.Laboratory screening tests on potato dextrose agar (PDA) showed that only two out of 16 fungi, namelyT. virideandT. harzianuminvaded the colonies ofCorticium rolfsiiand completely Inactivated the pathogen in culture. The colonies of the majority of the test organisms (75%) were overrun byC. rolfsiimycelium. Greenhouse studies showed thatT. viridecompletely inhibited the incidence of the basal stem rot disease of tomato when inoculated on the same day as, or three days before, inoculation withC. rolfsii, T. virideandA. flavusinoculated three days after the pathogen, did not significantly reduce disease incidence on tomato. However, disease severity was slightly reduced (53.3–58.8%) whenA. flavuswas inoculated on the same day as, or three days before, inoculation withC. rolfsii.During both dry and wet season trials in the field, disease severity indices (2·6–4·3%), were very low in plots in whichC. rolfsiiwas co—inoculated withT. viride.Much higher disease indices (88·7–97·1%) were recorded in plots in which the pathogen was co‐inoculated withA. flavus