A comparative study of two male recombination factors (31. 1 MRF and 23. 5 MRF) isolated from the same Southern Greek natural population, revealed specific differences in their activities. 23. 5 MRF induces female sterility due to atrophie ovaries at a wide range of temperatures while 31. 1 MRF does so only at high temperatures. The gross morphology of the atrophie ovaries was the same and unilaterally affected pairs were found in the F1 of crosses with both 23. 5 and 31. 1 MRF. Furthermore, 23. 5 MRF induces: (a) lower frequencies of abnormal anaphases I and II than 31. 1 MRF, (b) higher frequencies of ‘double crossovers’ resulting from deficiences or duplications, (c) large clusters of recombinants, suggesting premeiotic origin and (d) cases where one of the non-recombinant phenotypes was not produced. Such cases have never been observed with 31. 1 MRF. Moreover, the cytoplasm of the Cy L4/Pm strain that suppresses 31. 1 MRF does not affect the activities of 23. 5 MRF. Hypotheses to explain the different behaviour of the two factors are presented and discussed.