This paper presents an experimental test of a model that predicts the outcome of intraspecific competition between unit-restricted decomposer fungi. The model was tested using two isolates each of two species of the mushroom genus Coprinus (Basidiomycota). The fungi were grown on yeast extract agar at several resource densities. The effect of intraspecific competition was measured as the reduction of stipe dry weight (an indicator of spore production) when an individual grows in competition with a second individual of the same species as compared to the stipe dry weight when it grows alone. Additionally, competition often caused a significant change in the growth rate of the isolates. In general, the model accurately predicted the stipe dry weight of the isolates growing in competition, but it incorrectly predicted the outcome of competition for at least one resource density for each isolate.