The rates at which lignite is solubilized by the extracellular products of aPenicillium fungus designated as RWL-5, four substrains isolated from RWL-5 andStreptomyces flavovirens were examined. The effects of lignite surface oxidation were also examined. Comparison of data generated from studies with the two microorganisms suggested thatS. flavovirens solubilized coal in a more alkaline environment than that of thePenicillium strain RWL-5. The rate of biosolubilization by RWL-5 in an oxygen-enriched atmosphere was almost twice that in air. Two substrain isolates exhibited biosolubilization rates approximately two times as great as that of the parent strain.