The nematode P. autumnalis (TYLENCHIDA: SPHAERULARIIDAE) parasitizes M. autumnalis (DIPTERA: MUSCIDAE), sterilizing female face flies. This study examined how the instar of M. autumnalis affects the nematodes’ infection behavior and ultimate success. Two experiments were conducted: 1. we examined which instars were more acceptable to infective stage parasites for infection, 2. we examined what instars supported the nematode to complete its development. Based on the data collected above, we seek to understand the infection decisions of P. autumnalis. Researchers have recognized P. autumnalis, as a potential and safer alternative to current control methods for face flies. The effect of the nematode in natural populations might be increased with better knowledge of the factors that influence parasitism. These studies contribute to the overall understanding how abiotic and biotic factors affect spatial and temporal distribution of hosts and parasites.