To correlate changes of cell surface structure with the cell cycle phases, scanning electron microscopic observation was made on culture cell line of renal carcinoma of Syrian hamster (HKC-400) which was synchronized by double thymidine block. Cell surface morphology could be classified into three major categories depending on the cell cycle phases I.e. flattening with few excrescences in the S phase, many blebs and filaments in the M phase and active ruffling early G1. Further study is needed to characterize G2 and late-G1 cells. Functional significance was also discussed.