Envelope membranes were isolated by sucrose density gradient floatation centrifugation from the homogenate of cyanelles prepared from Cyanophora paradoxa. Two yellow bands were separated after 40 h of centrifugation. The buoyant density of one of the two fractions (fraction Y2) coincided with that of inner envelope membranes of spinach or plasma membranes of cyanobacteria. The other yellow fraction (fraction Y1) migrated to top of sucrose-gradient even at 0% sucrose. Pigment analysis revealed that the heavy yellow fraction was rich in zeaxanthin while the light fraction was rich in beta-carotene, and the both fractions contained practically no chlorophylls. Another yellow fraction (fraction Y3) was isolated from the phycobiliprotein fraction, which was the position where the sample was placed for gradient centrifugation. Its buoyant density and absorption spectra were similar to outer membranes of cyanobacteria. We have assigned fractions Y2 and Y3 as inner and outer envelope membrane fractions of cyanelles, respectively. Protein compositions were rather different between the two envelope membranes indicating little cross-contamination among the fractions.