A method was developed for the fractionation of Fusarium culmorum homogenates in order to obtain purified cytoplasmic membranes. The fractions obtained were characterized by studying their ultrastructure and enzymic properties. Since ATPase is an enzyme present in the cytoplasmic membrane, we studied the levels of this activity in our fractions as an indication of the presence of cytoplasmic membrane fragments. Since this activity is also present in mitochondria, we determined the levels of fumarase, a mitochondrial enzyme, and took the ratio of the two activities as an index of the purity of the membrane preparations. The sediment obtained by centrifuging the cell-free extract at 40000 g was fractionated in a discontinuous sucrose gradient. This led to three types of fractions. The most dense had a high ATPase/fumarase ratio and its ultrastructure showed that it contained membrane fragments having a triple-layered structure. We concluded that this fraction was rich in cytoplasmic membrane fragments; it was clearly distinguishable from the intermediate and less dense fractions. These latter fractions had a much lower ATPase/fumarase ratio and, judging by their ultrastructure, they were respectively a mitochondrial fraction and a fraction consisting of vesicles probably related to the endoplasmic reticulum. Analysis of the cytoplasmic membrane-rich fraction showed that it consisted of protein, lipid and 30 % carbohydrate