Walls of young hyphae of Mucor mucedo L. growing in the presence or absence of cytochalasin A were isolated and their chemical content determined. Cytochalasin A induced modified proportions of various monomers resulting in a reduction of the (neutral sugars + glucuronic acid)/glucosamine ratio. The walls contained less proteins but more chitin-chitosan and phosphate. These modifications are discussed in relation to ultrastructural changes described previously.