During 1955 and 1956 three parasites of the spotted alfalfa aphid, Therioaphis maculate (Buckton), were obtained from Europe and the Middle East. These parasites, Praon palitans Muesebeck, Trioxys utilis Muesebeck (both Braconidae), and Aphelinus semiflavus Howard (Eulophidae), were successfully mass produced in the insectaries at Albany and Riverside, California, and released in large numbers at many localities through. out the state. Initial efforts to effect parasite establishment in commercial alfalfa were greatly hampered by insecticidal treatment of the fields, interference from native coccinellids and fungus disease of the aphid, and the disruptive effects of the alfalfa harvesting process. However, heavy build-up of parasites in a controlled plot near Lancaster, California, in the summer of 1956, and the subsequent distribution of the parasites from this plot on cut alfalfa, resulted in widespread parasite establishment in southern California by the end of the year. In 1957 parasites were colonized in all T. maculata-affected areas of California, mainly with material collected by a large truck-mounted mechanical collector. By the end of 1957 parasites were established in areas involving about 1,000,000 acres of irrigated alfalfa and were playing an increasingly important role in the biological control of T. maculata.