Engorged females (EF) of Boophilus microplus tick were collected from Hereford, Criolla and Nelloreheifers of three different herds to study the influence of natural babesial infection on the following biological parameters of the EF: engorgement weight, preoviposition period, minimal egg incubation period, egg hatching and Reproductive Efficiency Index. No detrimental consequences to the reproduction of EF carrying babesial infection were found when compared with those EF in which no kinetes of Babesia were detected. Even the groups of EF most highly parasitized (21 to 110 kinetes per microscope field of hemolymph) showed biological values considered as normal, suggesting the existence of adaptive tolerance between B. microplus and the Babesia protozoa under field conditions.