ABSTRACTVery different microfungal species were obtained from forest soils in Rhode Island when two different isolation temperatures (0 C and 25 C) were used, even though isolates were obtained from the same samples. At 0 C, Zygomycetes, particularlyMucorandMortierellaspecies in the sectionsHygrophilaandStylospora, were favored. Most fungi isolated at 0 C were psychrotrophs capable of growing from 0 C to temperatures greater than 25 C. However, psychrophiles, mostlyMortierellaandMucorspp., were also isolated, and two newMortierellaspecies were identified. Isolations at 25 C resulted primarily in the recovery of Deuteromycetes andMortierellaspecies in the sectionIsabellina.Most of these isolates were mesophiles with growth minima between 5 and 10 C and maxima above 25 C. It is concluded that isolation temperature can seriously bias our perception of fungal species abundance, dominance and distribution in forest soils. Soil temperatures measured at 10 cm depth for 2 years in a Rhode Island forest remained at or below 10 C for 5 months of each year and did not rise above 19 C. Water potentials in the upper 15 cm of soil were greater than -0.4 MPa over the same period and were not limiting for the growth of most fungi. Therefore, low temperature fungi are potentially active throughout the year in soil of this temperate forest, whereas the activity of mesophiles may be restricted to periods when temperatures are above approximately 10 C. Isolations at 0 C should improve identification of fungi involved in decomposition processes at low temperatures.