The seasonal phenology of the walnut husk fly (WHF), Rhagoletis completa Cresson, was studied in the Willamette Valley of Oregon during a 4-year period (1990–1993). Adult emergence observed in ground emergence cages varied greatly among years. Initial emergence ranged from 29 June to 17 July, and last emergence occurred as early as 17 August or as late as 11 September. The period of adult emergence was constant from year to year (mean 55, range 50–60 days) with the peak between the 3rd week of July and the 1st week of August. The period between first observed appearance of females in ground-level emergence cages and the first oviposition was consistent among the study years, ranging from 20 to 23 days. Peak oviposition occurred in the 2nd and 3rd weeks of August. Mean egg hatch and larval survival were 98 and 80%, respectively. Infestations reached over 95% in unsprayed trees by early to mid September. Mature larvae first exited from husks beginning the 4th week of August and continued through mid October. Larvae penetrated the soil and pupated at a depth of 2.5–7.6 cm with most within the top 2.5 cm of soil.