The relationship between weights of the nymphal stage and their resultant sexes in Ixodes scapularis Say and Dermacentor variabilis (Say) was examined under laboratory conditions. Nymphs were fed to repletion on laboratory white mice, weighed individually, and held until they molted. Fully engorged I. scapularis nymphs that subsequently molted to females weighed between 3.8 and 6.4 mg (n = 64). Those nymphs that molted to males weighed between 2.0 and 3.2 mg (n = 57). Body weights of engorged nymphs that became females were significantly greater than those of nymphs that became males (P < 0.05). Similarly, the range of body weights of engorged D. variabilis nymphs that subsequently became females was 9.1-15.1 mg (n = 32), whereas those nymphs that molted to males weighed between 3.9 and 13.0 mg (n = 39). Body weights of engorged nymphs that became females were significantly greater than those of nymphs that became males (P < 0.05). This suggests that nymphs of both species that become females presumably imbibe more blood than those that became males.