The consumption of arbuscular mycorrhizal fungi (AMF) by Proechimys semispinosus (Central American spiny rat) was assessed via microscopic examination of faecal material for the presence of AMF spores. Mycophagy (indicated by the presence of spores) was compared among individual spiny rats residing on eight isolated islands in Gatun Lake, Panama during January and July 1996. Spores and sporocarps of Sclerocystis coremioides and spores from at least four species of Glomus were present in 77% of the 231 faecal samples examined. The proportion of faecal samples that contained AMF spores did not differ between spiny rat sex, age classes or months of sampling or with island area. However, there was a positive relationship between the proportion of samples containing spores and rodent density in January, and a marginally significant positive trend in July. Increased consumption of AMF on islands that supported high densities of spiny rats may have resulted from increased competition for primary food resources (fruits and seeds).