Chitin was separated inn lean form from skeletal pens of squids and shells by acid and alkali treatment, and acetylchitins of different degree of acetylation were prepared to investigate the film properties as semipermeable membrane. A fully acetylated chitin (degree of acetylation, 2) was prepared by the acetylation of chitin in the presence of perchloric acid with calculated amount of acetic anhydride at 0℃ for 5 hours. Acetylated chitins exhibited increased solubility in formic acid compared with chitin and diacetylchitin was easily soluble even in 85% aqueous formic acid. Acetylchitin films for ultrafiltration study were prepared from formic acid solution by casting on a glass plate and by solidifying in ethanol. Thus prepared films had wet thickness 10-l2μ, and dry thickness 7-8μ, indicating 25% water absorption. Rejection efficiency towards various solutes of different molecular weight including polyethylene glycols, saccharides, amino acid were examined and molecular exclusion limit was found to be about 7,500. This value is smaller than that of chitin films (∼10,000). Measurements of the distribution coefficient for the solutes correlates well with the difference in rejection efficiency toward solutes of similar molecular weights.