ABSTRACTPatterns of calcium oxalate crystal development on sporangiophores and sporangia ofGilbertella persicariaare a consistent feature of this fungus when grown on a variety of standard mycological media. When grown on a liquid synthetic medium consisting of dextrose and salts, initial growth of the fungus and production of calcium oxalate crystals mimics growth on standard solid media. Soon, however, crystal production becomes aberrant, then ceases. Concomitant with the change in crystal morphology, growth of aerial portions of the colony increases dramatically. Normal colony morphology and crystal production are maintained when calcium salts are added to the synthetic medium. Significantly more fungal dry weight is produced on the synthetic medium when no calcium is added.Data gathered suggest that: 1) mineralized deposits may provide structural support; 2) early production of crystal-bearing sporangia lowers calcium concentrations in the medium sufficiently to alter and eventually stop calcium oxalate crystal production; 3) calcium may inhibit growth; 4) calcium oxalate crystal production inG. persicariamay regulate environmental calcium concentrations.