Abstract. The locations of 77 markers along the chromosomal elements B (41 markers) and C (36 markers) of Drosophila subobscura, D. pseudoobscura, and D. melanogaster were obtained by in situ hybridization on polytene chromosomes. In comparisons between D. subobscura and D. pseudoobscura, 10 conserved segments (accounting for 32% of the chromosomal length) were detected on element B and eight (17% of the chromosomal length) on element C. The fixation rate of paracentric inversions inferred by a maximum likelihood approach differs significantly between elements. Muller's element C (0.17 breakpoints/Mb/million years) is evolving two times faster than element B (0.08 breakpoints/Mb/million years). This difference in the evolutionary rate is paralleled by differences in the extent of chromosomal polymorphism in the corresponding lineages. Element C is highly polymorphic in D. subobscura, D. pseudoobscura, and in other obscura group species such as D. obscura and D. athabasca. In contrast, the level of polymorphism in element B is much lower in these species. The fixation rates of paracentric inversions estimated in the present study between species of the Sophophora subgenus are the highest estimates so far reported in the genus for the autosomes. At the subgenus level, there is also a parallelism between the high fixation rate and the classical observation that the species of the Sophophora subgenus tend to be more polymorphic than the species of the Drosophila subgenus. Therefore, the detected relationship between level of polymorphism and evolutionary rate might be a general characteristic of chromosomal evolution in the genus Drosophila.Key words. Chromosomal elements, chromosomal evolution, chromosomal inversions, chromosomal polymorphism, Drosophila, Drosophila obscura group, evolutionary rate. The comparative analysis of chromosomal organization, mainly synteny and gene order, has been the focus of recent studies on chromosomal evolution. Changes in gene order are due to rearrangements affecting either small or large chromosomal segments (micro-or macro-rearrangements). After the sequencing of the whole genome of several organisms, both prokaryotes and eukaryotes, the analysis of gene order conservation is possible by direct comparison of complete genome sequences. The first genomewide comparative studies were performed between quite distant bacterial genomes and suggested a rapid evolution of gene order Tatusov et al. 1996). Later studies revealed that in yeast gene order evolution has been mainly shaped by small inversions including fewer than 10 genes (Seoighe et al. 2000). In eukaryotes with more complex genomes, the genomewide comparative studies have been extremely useful to detect groups of genes with conserved synteny, for instance, between humans and mouse (Kent et al. 2003;Pevner and Tesler 2003) or between Drosophila melanogaster and D. pseudoobscura (Richards et al. 2005).At another scale, the genome projects have facilitated the comparative mapping of multiple markers by in situ hybridization, es...