Employing extensive co-conversion data for selected and unselected sites of known molecular location in the rosy locus of Drosophila. we determine the parameters of meiotic gene conversion tract length distribution. The tract length distribution for gene conversion events can be approximated by the equation P(L > or = n) = phi n where P is the probability that tract length (L) is greater than or equal to a specified number of nucleotides (n). From the co-conversion data, a maximum likelihood estimate with standard error for phi is 0.99717 +/- 0.00026, corresponding to a mean conversion tract length of 352 base pairs. (Thus, gene conversion tract lengths are sufficiently small to allow for extensive shuffling of DNA sequence polymorphisms within a gene). For selected site conversions there is a bias towards recovery of longer tracts. The distribution of conversion tract lengths associated with selected sites can be approximated by the equation P(L > or = n/ selected) = phi n(1 - n + n/phi), where P is now the probability that a selected site tract length (L) is greater than or equal to a specified number of nucleotides (n). For the optimal value of phi determined from the co-conversion analysis, the mean conversion tract length for selected sites is 706 base pairs. We discuss, in the light of this and other studies, the relationship between meiotic gene conversion and P element excision induced gap repair and determine that they are distinct processes defined by different parameters and, possibly, mechanisms.