Demembranated indirect flight muscles of heterozygotes with Act88F6 demonstrate a pleiotropic effect upon the accumulation of actin-associated myofibrillar proteins, troponin-H, troponin-I, troponin-T and tropomyosin are missing, actin levels are normal. The myofibrils of heterozygotes show the presence of large masses of fibrillar-like material which is not organised into discrete myofibrils.