The metaphase and salivary gland chromosome groups of Drosophila robusta are described. Metaphase complements of males and females are alike, showing 3 pairs of V's of decreasing sizes and a pair of microchromosomes. The largest V's are the sex chromosomes. In addition to the 6 standard salivary gland chromosome arms, 13 other gene arrangements are known : 3 in XL, 3 in XR, 4 in 2L, 1 in 2R, and 2 in the third chromosome. All are due to inversions, of which 2 are terminal and 2 are pericentric. One of the latter causes a detectable change in metaphase chromosome shape from a V to a J. Of these 19 arrangements, 4 are rare, each having been found in a different single locality only. Most of the other sequences are widespread throughout the range of the species (eastern United States), but display gradients in frequency. These gradients are most clear from north to south; in some cases either complete dominance or the extinction of an arrangement is encountered. The pattern of geographical distribution of the gene arrangements is discussed and an attempt is made to relate modern distribution with the presumed past history of the species.