Rat derived Pneumocystis carinii cultured in vitro was analyzed by a variety of light microscopic and staining techniques for identification of developmental stages and changes in these populations over time in culture. Evaluation of culture dynamics based on a limited number of life cycle stages indicated that trophic replication contributed significantly to the observed growth in vitro. Identification of additional life cycle stages, particularly early cyst forms, suggested that the process of maturation from early precyst to mature cyst may also contribute to the net growth. All developmental stages identified from culture were verified by their presence in stained impression smears of infected rat lung. Based on these data, a tentative life cycle for P. carinii in vitro has been proposed.