Erythrocyte densities in the blood meals of six Anopheles mosquito species were compared with those of human host erythrocyte densities. During engorgement, An. gambiae Giles and An. stephensi Liston concentrated erythrocytes by factors of 1.8 and 1.7, respectively; An. freeborni Aitken did not concentrate; and An. arabiensis Patton and An. dirus Peyton & Harrison demonstrated an intermediate level of erythrocyte concentration (1.4 and 1.2, respectively). An. albimanus concentrated host hemoglobin, but hemolysis during engorgement decreased bloodmeal erythrocyte density below that of host blood. The degree to which anopheline species concentrated erythrocytes was related to the frequency and time spent undergoing prediuresis (anal excretion of fluid during feeding), suggesting that prediuresis is responsible for erythrocyte concentration and that the fluid produced represents efflux from the filtration of ingested blood. Differences observed in erythrocyte concentration by different anopheline species are consistent with species-specific patterns of host selection.