Four fracture faces of Codium australicum (Silva) chloroplast envelopes are characterized in the alga and after incorporation into Elysia maoria (Powell). The outer membrane generally has a lower density of particles than the inner in both alga and animal. Both inner and outer envelope faces in the animal are significantly different to those in the alga in terms of particle size distributions. The outer envelope also differs in terms of particle densities. No evidence was found for a phagosome membrane surrounding the chloroplast in the animal. The chloroplast surface area is the same in the alga and in the animal, but the plastid volume increases in the animal. Changes in the envelope membranes may be related to functional aspects of the chloroplast-sacoglossan association.