Between November 1998 and April 1999, all new felons committed to the California Department of Corrections were classified under the old and new classification systems. The designation to a prison was randomly determined by whether the identification number assigned to an inmate was even or odd. Inmates who received odd identification numbers were designated with the score from the new classification system, and inmates with even numbers were designated by the old classification system. This resulted in 561 inmates being classified as Level I under the old system but Level III under the new system. A total of 297 inmates went to Level I prisons, and the other 264 inmates were committed to Level III prisons. The behavior of the inmates was tracked for 2 years, and most inmates remained in custody for this period. There were significant differences in custody and security practices from Level I to Level III, with housing being more restrictive in Level III. It is reasonable to assume that inmates in the study who served time in Level I facilities were surrounded by inmates with less serious criminal histories than the inmates serving time at Level III prisons. Findings show, however, that the percentage of inmates with misconduct did not significantly differ between Level I and Level III. This study provides some evidence that the types of inmates housed together may not be as important as often believed, or at least that the effect can be overcome with adequate security and custody measures, along with other environmental influences. Neither is inmate misconduct more prevalent in higher security prisons. 2 tables, 2 figures, and 33 references