The microwave spectrum of the methylcyclopropane-HCl (MCP.HCl) complex has been observed using a pulsed nozzle, molecular beam Fourier transform microwave spectrometer. Transitions were assigned with the aid of the quadrupole hyperfine structure from the chlorine nucleus. The rotational constants of the C4H7.(HCl)-Cl-35 species are A = 6408.1(2) MHz, B = 1410.486(1) MHz, and C = 1224.155(1) MHz. In addition to this species, the spectra of the MCP.(HCl)-Cl-37 and MCP.(DCl)-Cl-35 isotopomers have also been studied. The data are consistent with a structure in which the HCl interacts with the C-C bond of the MCP ring that is adjacent to the methyl-substituted carbon.