A myofibrillar protein extract has been isolated from the muscle of Ascaris suum. Two-dimensional electrophoresis of this extract revealed that the myosin light chain 1 (ALC 1) migrates as 3 components with approximate isoelectric points in the range of 5.3–5.6. The most acidic component of ALC 1 appeared to be phosphorylated when the myofibrillar extract was incubated for 10 s with catalytic subunit of cAMP-dependent protein kinase and [γ- 32P]ATP. The myosin light chain 2 (ALC 2) migrated as a single component in isoelectric focusing with an approximate isoelectric point of 5.5. Actin was resolved into 2 components with identical molecular weights but isoelectric points differing by approximately 0.2 pH units. A protein was tentatively identified in the myofibrillar extract as tropomyosin. It migrated as a single band with an approximate isoelectric point of 5.0 and a molecular weight of 39 000. None of the troponin components could be identified in the myofibrillar extract. It is postulated that muscle contraction in A. suum muscle could be controlled by phosphorylation of myosin.