Samples of 8 natural populations of Drosophila pseudoobscura inhabiting northern Mexico were cytogenetically analyzed. Twelve distinct chromosomal arrangements of the third chromosome were identified. Number of inversions at each locality varied from 6 to 8. A total of 468 third chromosomes were examined in the area of study, and relative frequencies of inversions in each population were calculated. The most common inversions in decreasing order of relative frequency were: TL, SC, OL, and CU; the other 8 inversions had frequencies <10%. Each locality had 3–4 inversions that accounted for ≤80% of the total. All populations were heterogeneous. Relative frequency of each inversion did not form a gradient along the transect studied. Unexpected was the presence of the ST inversion, as well as the incidence, in some populations, at a moderate frequency, of the inversion OL. Another exception was the increase in frequency of the EP inversion compared with previous collections in the area. More information from the region is needed.