Proteolytic activity was demonstrated in each of the midgut lumen content (MLC) and midgut wall (MW) of the mated unfed and fed female and male Ornithodoros erraticus. In both midgut compartments proteinase activity levels increased after feeding to reach at their peaks about double times the levels in the unfed controls. The peak levels of the proteolytic activity were attained on 1 and 1 – 2 days after feeding (daf) in the MLC (48 and 31.10 – 32.63 µg alanine ×10³/ min/ mg gut content) and on 5 – 6 and 5 – 7 daf in the MW (196.33 – 200.33 and 156.80 – 161.23 µg alanine ×10³/ min/ mg tissue) in the female and male tick, respectively. However, the proteolytic activity in the MLC was much lower than that in the MW of all the corresponding physiological states studied, and represented about one fourth and one fifth of it at the peak activity levels in the female and male tick, respectively. Generally, proteolytic activity was higher in the female than male ticks in the unfed and up to the 2nd daf in the MLC and in all the corresponding examined states in the MW throughout the period of study (20 daf).     Changes in the level of proteolytic activity in each midgut compartment of the female and male ticks infected with Borrelia crocidurae followed almost the same pattern in the uninfected controls. However, the level was lower in the MW of Borrelia infected ticks than uninfected ones throughout most (2–20 daf) of the period of study.     The present results were discussed and conclusions conformed to the view of intracellular proteolytic digestion of blood meal in the MW, but did not exclude a possible role of extracellular proteolytic digestion in the MLC.