The most-studied organocopper, methylcopper (CH3CU), has been synthesized for the first time as a monomer, free of other ligands or coordinated solvent, and detected using millimeter-wave rotational spectroscopy. The molecule was created in the gas phase by the reaction of copper vapor and several different precursors, including CH4 and tetramethysilane. An obvious symmetric top pattern was observed, indicative of C, symmetry. For confirmation, rotational spectra of four isotopically substituted species, (CuCH3)-Cu-65-C-12, (CuCD3)-Cu-63-C-12, (CuCH3)-Cu-63-C-13, and (CuCH3)-Cu-65-C-13 Were recorded as well, following large-scale syntheses of specifically labeled precursors. From this data set, accurate spectroscopic constants and a structure for CH3Cu were derived. The r(m)((2)) geometry shows the shortest alkyl carbon -copper(I) bond length known (1.8809 Angstrom) and a rather large H-C-H angle in the methyl group (109.88degrees). The measured rotational and distortion constants are well reproduced by QCISD calculations. This study should allow further refinement of theoretical calculations on methylcopper and other organocopper derivatives, which are valuable synthetic intermediates in organic chemistry.