A systematic random sample of prisoner files was selected from the records of the inmate population of the United States Disciplinary Barracks (USDB), a maximum security prison at Fort Leavenworth, Kansas that houses prisoners for the U.S. Army, Air Force and Marines. The offense, sentence, release date, and custody at time of release for each subject was extracted from the files. Data were also collected on Disciplinary reports on-the-spot reprots of lesser offenses, and on-the-spot reports of positive extraordinary. Other variables were custody grade acheved and imprisonment time. Data analysis suggested a strong tendency for long-term inmates in both high risk and low risk custody groups to commit rule infractins at midpoints in their institutional careers. Bad-on-the-spot reports showed the same tendency to increase in frequency during the middle stage of confinement; but goo-on-the-spot reports became less and less likely as release date neared. Higher ranking guards issued more infraction reports during the middle stage of confinement than did lower in the overall trend of inmate midphase nonconformity. It is concluded that conformity to subcultural norms of the prison environment is strongest in the middle stage of confinement. Also, the unique dynamics of interaction between custodial perosnnel and inmates in individual institutional settings exercise a strong effect on this pattern. Forty-eight references, an appendix presenting additional data 12 tables, and 5 figures are provided.