In the late summer of 1949 cases of what was generally assumed to be abortive or non-paralytic poliomyelitis occurred in and around Worcester, Massachusetts. In cooperation with the Division of Communicable Diseases of the Health Department of the Commonwealth of Massachusetts, epidemiological and laboratory studies were initiated in an effort to determine more precisely the etiology of the disease. The recent investigations of Dalldod (1) and of Melnick (2--4) and their collaborators have resulted in the isolation of numerous virus strains from patients suffering from illnesses resembling abortive poliomyelitis in various parts of the country. These agents are characterized by their limited pathogenicity for laboratory animals, since suckling mice and hamsters and immature cynomolgus monkeys (5) are the only species proven to be susceptible so far. Most of the strains produce lesions in the skeletal musculature, leaving the CNS and other viscera unscathed. With other strains, lesions involving the brain and the myocardium and other visceral organs have been mentioned, though not described in detail. Serological evidence suggests the existence of at least several apparently distinct types (6-8). Since the clinical features of the Worcester cases resembled those exhibited by the patients from whom Dalldorf et al.(1) and Meinick et al. (2-4) had isolated their strains, attempts were made to isolate a similar agent from the Worcester cases. Two viral agents were isolated and propagated in series. The present communication deals with the isolation of one agent and with a description of its more obvious biological properties. Of considerable interest are the unusual pathological lesions which it causes in animals, and these are described in some detail in the accompanying paper (9). In addition its serological relationship to members of the so called Coxsackie group (10) of viruses, and certain other agents has been investigated.