Two samples of offenders from the Wayne County Prosecutor's Office in Detroit were used. The first sample consisted of 647 career-criminal cases disposed of in 1975, 1976, and 1977, while the other sample consisted of 408 noncareer-criminal felony cases processed in the same jurisdiction during the same period. Demographic, current offense, and detailed prior-record information were collected for all cases. Results of cross-tabular and regression analyses showed that while the career criminals were more violent, these differences in behavior patterns accounted for only 20 percent of the variation in the decision to prosecute or not prosecute an individual as a career criminal. More specific analyses of criminal behavior patterns showed that individuals prosecuted as career criminals had a pattern of committing more serious crimes for a longer period of time and specialized in committing more serious crimes. Persons prosecuted as career criminals can best be described as semiprofessional criminals. Figures, tables, chapter notes, and about 70 references are supplied. Study data analyses are appended. (Author abstract modified)