Relative frequencies of heterozygous inversions in a natural population of Drosophila willistoni from a locality of Veracruz, Mexico, were determined during the period from January 1994 to January 2001. An analysis of 636 chromosomes was performed on smears of salivary glands. Of the 70 known inversions present in the species, I detected only 45 of them, representing 64.3% of the total. Most of these 45 inversions had frequencies <10%. Six were found only once in the sampling period; the remaining appeared more than twice, with an erratic presence and variable relative frequency. Inversions A, F, and G of the XL chromosome arm were present in all samples, as was inversion E of the IIR chromosome arm and inversion B from the third chromosome. The inversions that attained average frequencies >10% were inversion A (chromosome XL), inversion E (IIR), and inversion B (III). Changes in the relative frequencies in some inversions were detected for the period studied. The average number of heterozygous inversions per female ranged from 1.92 to 3.77, with an average for the entire sampling period of 2.57. Thus, this Mexican population of D. willistoni possesses a considerable degree of chromosomal polymorphism.