ABSTRACTIntraspecific hyphal interactions were tested with five species of aquatic hyphomycetes. When inocula were introduced at a distance of 2.5 cm, growth of both strains of interacting pairs slowed down as they approached each other. Growth reduction was negatively correlated with geographic distance between the isolation sites of interacting strains. In every interaction, at least a few hyphae continued growth and intermingled with hyphae of the opposing colony. When inocula were placed adjacent to each other, the two strains intermingled without evidence for hyphal interference. In 6 out of 74 combinations, a very slight increase in aerial mycelium was observed.