The spatial and temporal distribution of onion maggot (OM)-induced plant damage were studied both within and among onion fields. Distribution of damage within a field of onions was found to be random in untreated plots and aggregated in treated plots early in the season when damage was low. The distribution of plant damage departed significantly from random in all plots later in the season as damage increased. Aggregation or clumping was found to predominate in the regional analysis of plant damage among fields. There were primarily two periods of OM damage during the season, and the first generation of OM larvae caused 65% of the total damage. Possible reasons for the aggregated distribution of damage both within and among onion fields are discussed.