Studies with reagents that crosslink two thiol groups have shown that it is possible to trap nucleotides at the active site of myosin chymotryptic subfragment 1. Subfragment 1 incorporates nearly stoichiometric quantities of [14C]ATP or [14C]ADP in a manner that depends linearly on the extent of inactivation by either N,N'-p-phenylenedimaleimide or Co(II)phenanthroline/[Co(III)(phenanthroline)2CO3]+ complexes. The incorporated radioactive nucleotide is retained after gel filtration, even when the enzyme derivatives are stored in the presence of EDTA or nonradioactive nucleotides (t 1/2 approximately 5 days). The nucleotide incorporated is not covalently bound because HClO4 denaturation allows immediate release of bound nucleotide. The nucleotide retained is ADP because the gamma-phosphate of [gamma-32P]ATP is lost after trapping. Subfragment 1 inactivated as above does not bind the competitive inhibitor adenosine 5'-[beta, gamma-imido]triphosphate, indicating that the active site is blocked. It is proposed that a jawlike nucleotide cleft closes on MgADP or MgATP, which can be locked shut by crosslinking two thiol groups by reaction with N,N'-p-phenylenedimaleimide or cobalt phenanthroline complexes.