Acetylcholine receptors from fish electric organ tissue and mammalian muscle were compared by peptide mapping. The alpha subunits from receptors of Torpedo californica and Electrophorus electricus electric organ tissue were digested with V8 protease and the resulting fragments separated on polyacrylamide gels and stained for protein or for carbohydrate. 125I-Labeled alpha subunits of acetylcholine receptors from Electrophorus electric organ tissue and bovine muscle were digested with V8 protease, and the resulting fragments were also separated on polyacrylamide gels. Intact receptors from both the fish electric organs and mammalian muscle were labeled with [4-(N-maleimido) benzyl]tri[3H]-methylammonium iodide which binds specifically to the acetylcholine binding site on alpha subunits, and the isolated alpha subunits were subjected to the same peptide mapping procedure. The fragment patterns produced were stained for protein and fluorographed to identify active site containing polypeptides. None of these peptide mapping approaches revealed extensive homologies between alpha subunits. Intact and V8 proteolyzed sodium dodecyl sulfate denatured receptors from Torpedo and Electrophorus electric organs and bovine muscle were electrophoretically transferred to diazophenyl thioether paper and probed with antisera to Torpedo receptor subunits and two monoclonal antibodies. Unique fragment patterns were obtained with each antisubunit serum. A fragment of the same size was derived from the beta subunit of each acetylcholine receptor and was shown to specifically bind the same monoclonal antibody in each case. These results indicate that only in the beta subunits from all of the species examined is a large sequence nearly identical. However, it is likely that corresponding receptor subunits from receptors of all of these species have extensive structural homologies.