Artificially infected adult flies were used in this study. In both Musca species, P. bovicola developed in the fat-body cells, mainly of the abdomen. Escape from the midgut and penetration of the fat-body cell was possibly achieved by use of the cephalic hook of the microfilaria. At 27 degrees C, development to the 3rd larval stage took 9 days, but maximum length was only reached after 11 days. Sharp increases in larval length took place on Days 6-7 and on Day 9. These sudden increase possibly indicate moults to the 2nd and 3rd larval stages respectively. During larval development the fat-body cell increased markedly in size to from a thin-walled capsule around the larva. On reaching the 3rd stage, larvae escaped from the capsules and migrated to the head cavity and proboscis.