Spontaneous 2nd chromosome recombination was observed in males of D. melanogaster newly derived from natural populations. From a Cambridge, England, natural population, 2 lines out of 20 showed male recombination, and in an Oklahoma, USA, natural population all of 25 lines showed male recombination. One line, OK1, was characterized in more detail. Male recombination was probably caused by dominant 2nd and dominant 3rd chromosome elements. The majority of recombination events in the 2nd chromosome occurred in the centromeric region and in the right arm, and male recombinants were recovered in clusters, suggesting that some male recombination occurs in premeiotic cells. Since both single and multiple recombinants were recovered, it was possible to measure chromosome interference for male recombination. This interference was observed to be negative. Finally, there is a reciprocal-cross effect associated with male recombination and F1 female sterility in the OK1 line. This effect on male recombination is apparently not caused by a cytoplasmic suppressor of recombination. The genetic basis and mechanism of male recombination are discussed, along with its possible influence on adaptability in natural populations.