Collections of Anopheles darlingi Root and An. albimanus Wiedemann from central and northern Belize were conducted as landing captures from 6:30 to 8:00 p.m. to define spatial distributions and outdoor:indoor ratios of biting during the early evening. In central Belize, collections were made at 31 houses in riparian zones (> or = 1 km from rivers) and 14 houses in upland zones (>1 km from rivers) during the dry and wet seasons of 1993 and 1994. Females of both species were abundant in houses < or = 1 km from rivers. Females were not present in houses located in upland areas during the dry season, but were present in the wet season. A total of 63 paired collections (representing 130 individual captures) from 42 houses showed An. darlingi females were more endophagic (ratio of 1:0.6) during the early evening than were An. albimanus females (ratio of 1:0.21). Paired landing collections from 22 houses in riparian zones in April-May were analyzed in an index of species abundance (ISA). ISA values rated An. darlingi as the dominant Anopheles mosquito indoors and An. albimanus was dominant outside. Although An. darlingi and An. albimanus were abundant in riparian zones, there was no association in their numerical abundance, suggesting that different environmental factors influenced their abundance. In northern Belize, one house for each of 16 villages was sampled during April and May 1994. Large numbers ofAn. albimanus were captured outdoors in houses located in riparian and marshland areas (means of 217.5 and 247.5/1.5 personhours outdoors, respectively). Numbers of An. albimanus were low at houses located away from rivers and marshes (12.2 per collection). Anopheles darlingi was uncommon at sites in northern Belize. Proportionally fewer An. albimanus females entered houses in the north (outdoor:indoor ratio of 1:0.16) compared to the central region (ratio of 1:0.21), which probably reflects differences in house construction, anti-mosquito behavior (i.e., closing windows and doors at sunset), and insecticide treatments. The ISA gave a quantitative assessment of vector dominance in relation to the parameters of spatial distribution and numerical abundance. The index was also sensitive to the variables of indoor and outdoor biting behaviors.