In a paper published seven years ago in this journal (Coyne and Orr 1989a), we analyzed the time course of speciation in Drosophila by correlating electrophoretic genetic distance between pairs of species (a number roughly proportional to their divergence time) with the strength of reproductive isolation between them. That analysis yielded five conclusions. First, both prezygotic and postzygotic reproductive isolation increase with divergence time between taxa. Second, prezygotic (sexual) isolation evolves more rapidly than postzygotic isolation (sterility and inviability of hybrids). This difference is, however, due entirely to much stronger prezygotic isolation between sympatric than between allopatric pairs of species. We suggested that this difference was due to reinforcement, or direct selection for sexual isolation that occurs among sympatric taxa that produce unfit hybrids (Dobzhansky 1937). Third, hybrid sterility and inviability evolve at similar rates. This conclusion now appears to be incorrect because average divergence time between taxa is not a sensitive way to measure evolutionary rates of reproductive isolation, and more sensitive analyses show that hybrid sterility may in fact evolve more rapidly than inviability (Wu 1992). Fourth, the usual pathway for the production of postzygotic isolation is the initial appearance of sterility or inviability in hybrid males, followed by its appearance in females. This explains the frequent observation of Haldane's rule: the pattern that if only one gender of hybrids is sterile or inviable in species crosses, it is nearly always the heterogametic (XY or XO) sex (Haldane 1922; Coyne and Orr 1989b). Finally, there is a large increase in genetic distance between those species pairs producing sterile or inviable males only and those producing sterile or inviable hybrids of both sexes. This implies that there is a long time lag between the evolution of postzygotic isolation in males and in females. While a similar (but much smaller) analysis has since been conducted in salamanders (Tilley et al. 1990), the data from Drosophila are unique-and are likely to remain so-because of the large number of crossable species and the ease of estimating sexual and postzygotic isolation in the laboratory. These Drosophila data have hence attracted some interest. Because of this, we have continued to accumulate new data as they have appeared. We have also found a few errors in our original data set, and have revised some estimates of reproductive isolation and phylogenetic relatedness when better data became available. We now have data for 171 interspecific hybridizations in Drosophila, an increase of 43% over the 119 hybridizations described in our previous paper. Because DNA sequencing has largely supplanted gel electrophoresis as a way of measuring divergence between species, it is unlikely that this data set will grow much larger; and it will be many years before we possess DNA-based estimates of divergence between many pairs of Drosophila species. We therefore thought it timely to check our earlier conclusions using the new and larger data set.