Adult and immature populations ofAgromyza frontella(Rondani) (Diptera: Agromyzidae) were monitored in 2 alfalfa fields in central PA. Four generations per year were found in 1977 and 1978. Two sampling methods, the sweep net and the vacuum net, compared favorably in estimating relative adult abundance. Adult sex ratio ranged between 50–100% female, varying significantly with sample date. Effective population numbers, i.e., the numbers of individuals leaving progeny, were much lower than actual population numbers. Sampling of eggs and larvae showed slight correlation of eggs with plant height, patchy distribution, and seasonal trends. Rearing of field collected pupae showed wide variations in percent emergence, sex ratio, and population abundance from generation to generation. Ten species of parasites were reared, including 3 new host records and 3 possible new species.