Three different strains ofOphryotrocha labronicawere used for all the possible reciprocal crosses. The results substantiate Bacci's thesis (1978), according to whichOphryotrocha labronicawould consist of sexual races with different valences in geographically distinct populations, and indicate a reproductive isolation on ethological bases. A close correlation has been found between the occurrence of sexual races and the difference in the degree of courtship activity according to strains.