Previous results (Siemankowski, R.F. and Zobel, C.R. (1976) J. Mechanochem. Cell Motility 3, 171--184) demonstrated that, relative to myosin from rabbit skeletal muscle, myosin from lobster abdominal muscle has four times as many sites susceptible to tryptic fragmentation at the fast rate. The present studies show that limited tryptic digestion of lobster myosin results in the rapid production of three species of rod fragments, all of which are insoluble at low ionic strength; a subfragment-1-like species; and, in addition, the release of large amounts of small peptides (35%, w/w). From estimates of the yields of the tryptic fragments, it is found that although 1 mol equiv. of rod-fragment is produced per mol of myosin digested, only 1 mol equiv. of a subfragment-1-like species is found, suggesting that the lobster myosin subfragment-1-moiety is much more trypsin-labile than the analogous region of rabbit myosin. By two-dimensional electrophoretic analysis, it was found that two of the rod species comprise a large number of unique peptides, collectively, after denaturation in 9 M urea. Similar analysis demonstrates that the subfragment-1-like species contains a small (Mr 25 000), very basic peptide, and that during digestion with trypsin the larger light chain (Mr 20 000) is converted entirely into a more acidic light chain fragment (Mr 18 500). The smaller light chain (Mr 16 000) is resistant to tryptic proteolysis.