Subunits 70S, 50S, and 30S of ribosomes of E. coli and T. maritima have been studied by atomic force microscopy. A considerable heterogeneity of structures was visualized when 70S and 30S subunits were sorbed on mica. The linear size and the height of molecules were estimated. It was found that the heights of ribosomes of E. coli and T. maritima substantially differ. The average height of 70S ribosomes of E. coli was 9.4 + 0.01 nm and that of T. maritima was 10.35 +/- 0.02 nm. The differences in the dimensions were probably determined by special organization of the mobile ribosomal element the L7/L12-stalk.