We studied clinal variation in karyotypes of Peters' tent-making bat (Uroderma bilobatum) along a transect from south-central El Salvador to northwestern Nicaragua. Nondifferentially stained karyotypes were scored for 291 specimens from 5 localities. Each locality was near a locality from which similar data on U. bilobatum have been reported in the literature 18 years ago. G-tests indicated no significant differences between our data and the historical data at 2 localities in El Salvador; significant differences were found at the contact zone in Honduras. Based on a combined sample of 834 specimens, by means of the point at which genotype frequencies equal 20% and 80% (20/80 rule), we estimated that 3 chromosomal morphs (designated Aa, Bb, and Cc) had an average clinal width of 33.3 km. The widest cline, Bb, was 12.53 km wider than the narrowest, Cc. Morph Cc was detected at 2 localities, 72 and 80 km northwest of the nearest locality from which it was previously known. Overall, we detected little change in clinal architecture during the intervening years between these 2 analyses. Four investigators who used the same data set have published 4 different interpretations of clinal dynamics. These interpretations are mutually contradictory and so all cannot be correct.