Hyperparasitism is common in laboratory colonies of Egyptian Ornithodoros erraticus whether the ticks are on or off the rodent host. Sex, recent engorgement, and size appear to be major factors in this feeding relationship. Males, and nymphs that produced males (N-male), parasitized females and nymphs that produced females (N-female) more frequently than they parasitized males and N-male. Females and N-female seldom parasitized females and N-female and did not parasitize males or N-male. Engorging and recently engorged, large ticks attracted smaller unfed ones. No preferred attachment site was observed. Hyperparasitizing females and parasitized females weighed less, had a longer preoviposition period, and produced fewer eggs than normally feeding, unparasitized females. About 30% and 80% of the females parasitized by males and females, respectively, died within 3 mo; only 7% of unparasitized females died within the same period. The proximity of replete or engorging ticks may be sufficient stimulus for hyperparasitism. Additional factors may include production of an attracting kairomone by fed ticks or the absence under certain conditions of a deterrent to hyperparasitism normally produced by fed ticks. Borrelia crocidurae spirochetes are transmitted during hyperparasitism.