Electrophoretic and cytogenetic studies were undertaken on the population structure of Anopheles albimanus from 11 localities in Colombia, 3 from northern (Atlantic coast) and 8 from southern (Pacific coast) regions. Of the 25 allozyme loci examined, significant allele frequency differences were observed at 4 loci: hydroxy acid dehydrogenase (Had-1) and 3 esterases (Est-2, Est-4 and Est-6). The northern populations had higher variability, with 55% polymorphic loci, a mean heterozygosity of 20.4% and a mean of 3.0 alleles per locus. These values for southern populations were 24%, 9.1% and 1.5%, respectively. There were neither diagnostic loci nor clinal effect on frequencies of allozymes. Except for a small inversion on the X chromosome in low frequency in certain populations, all populations were homosequential in chromosomal banding patterns. Hybrids from matings between natural populations and the Gainesville laboratory strain were fully fertile. Estimates of genetic similarities (0.95-0.97 among southern and 0.99-1.00 among northern populations) suggest a lack of significant genetic differentiation among distant populations in this species. Based on the chromosomal, hybridization and electrophoretic data, we concluded that mosquitoes from the 11 collections were conspecific populations of An. albimanus.