The growth rate and electrophoretic pattern of r-proteins extracted from ribosomes of T. novellus grown under autotrophic and heterotrophic conditions were investigated. When T. novellus was grown in the autotrophic medium, cells entered the log phase at the end of the first day after which the growth continued for 3 to 4 days until the stationary phase is reached. In the heterotrophic media the growth proceeded at a much faster rate and cells entered the stationary phase 21 h after inoculation. Major differences were detected when the r-proteins of T. novellus grown either as an autotroph or heterotroph were analyzed by disc gel electrophoresis. The results showed that only 5 protein bands were common to ribosomes derived from both types of cells, and that 9 to 10 protein bands were specific only for one type of ribosomes. This indicates that the major modification of protein complement of ribosome structure must occur when cells adapt to different cultural conditions. It was suggested that when cells are grown in different media, the rates of synthesis of macromolecules are substantially altered resulting in heterogeneous ribosomes.