Following its invasion of eastern Ontario during the mid-1970's, numbers of the alfalfa blotch leafminer,Agromyza frontella(Rondani), increased rapidly to economic proportions and then receded to near tolerable levels. Thirty-three life tables were compiled from population and mortality data collected during this colonization phase. Analysis of the components of mortality using a variance/covariance matrix of the separate mortalities, as expressed askvalues, showed that maggot mortality within leaf mines was the key factor that determined population trends. Maggot mortality was the result of two contemporaneous processes fluctuating with a similar order of magnitude: exploitation and interference competition among instars, and predation by a complex of indigenous nabids and mirids. Although the former process was density dependent, its regulatory potential was damped by the inverse response of the predators, whose activity diminished the competitive interaction. Prepupal mortality was high but relatively invariant. Mortality from other causes was low and contributed little to population trend. Life tables for the larval stage in central Quebec resembled those for Ontario.