This study investigated principals' assimilation techniques and work behaviour of newly employed teachers in private secondary schools in Rivers State, Nigeria. Two research questions guided the study, while two hypotheses were tested. The study adopted correlation design. The population of the study comprised 389 principals and 463 newly employed teachers serving in the private secondary schools in Rivers State. A sample of the study was 172 elements comprising 86 principals and 86 newly employed teachers in private secondary schools in Rivers State. The first and second instruments used were self-designed questionnaires titled "Principals' Assimilation Techniques Questionnaire' (PATQ) and 'Teachers' Work Behaviours Assessment Questionnaire' (TWBAQ). The instruments were validated and subjected to reliability tests, in which indexes of 0.82 and 0.84 were obtained using test-retest and Pearson Product Moment Correlation (PPMC) techniques. The data generated were analysed using Regression Statistics to answer research questions and to test the hypotheses at 0.05 alpha level. The findings of the study revealed that positive and strong relationship existed between induction and work behaviour of newly employed teachers in private schools in Rivers State. The study further revealed that positive and weak relationship existed between supervision and work behaviour of newly employed teachers in private secondary schools in Rivers State.Recommendations made included that School managers should re-evaluate the modalities used in supervising newly employed teachers with a view to improving its processes to foster assimilation of newly employed teachers. The study also recommended that school leadership should ensure that adequate supports are given to newly employed teachers during their probation period. In addition, administrators should assess the strengths and weaknesses of newly employed teachers in order to determine the areas they need supervisory supports to improve on their performance abilities in classroom.