Warbles induced by the rodent bot fly larvae, Cuterebra fontinella, developed over a period of 3.5-4 weeks in the deer mouse Peromyscus maniculatus. Warble location, structure and dynamics were examined. Clustering of warbles occurred primarily within the inguinal region of the deer mouse with a significant number favoring the area between the anus and genital organ. No differences in warble production were found in hosts of either sex. On two occasions, a warble was encountered forming directly in the genitals. Warble structure consisted of a pore, cavity and capsule, all of which changed in shape and size as the larvae grew. A new pore was formed during the first larval molt and was commonly bordered by a yellowish precipitate. A bi-layered warble capsule surrounded the cavity as a thin layer and a thick diffuse outer layer. The growing warble expanded into the subcutaneous tissue of the inguinal area and stretched the hide caudally. Subcutaneous tissue surrounding developing warbles became relegated to small areas, leaving only a thin muscle to separate the warble from the peritoneum. Widening of the pore at the time of larval emergence is described. Linear relationships are given describing warble growth and pore enlargement as a factor of time. The location of warbles and the morphology of the pore are discussed in relationship to larval protection. Damage to the host due to warble formation is also addressed.