ABSTRACTThe effect of environmental and cultural conditions on the growth and competition between shiitake mushroom mycelium and the competitorTrichoderma harzianumwas investigated.T. harzianumhad higher growth rates and temperature optimum and tolerated more variable pH.T. harzianumgrew at lower water potentials than didL. edodes.No difference was detected in growth or dual challenge culture under moderate levels of CO2, pre-exposure toT. harzianum, low levels of sucrose and KCl supplementation, and substrate moisture contents between 39% and 70%. Conditions that favored the growth ofL. edodesoverT. harzianumwere moisture contents above 35%, 2 or 4 h of autoclaving the substrate, the use of oak sawdust in place of a hickory-ash mixture, the use of weathered sawdust, supplementation with 4% gypsum, low levels of bran supplementation, partial replacement of organic nitrogen sources with urea, and low levels of straw supplementation. Shiitake strain 465 exceeded strain 305 in its ability to compete withTrichodermawhen grown on a 9% bran supplemented medium. A simple technique was developed forin situobservations of growth and fungal interactions.