We report angle-resolved photoemission spectra both above and below T c in the single-plane cuprate superconductor Bi 2 Sr 22x La x CuO 61d . The superconducting state measurements show a highly anisotropic excitation gap with a maximum magnitude smaller than that of the bilayer compound Bi 2 Sr 2 CaCu 2 O 8 by a factor of 3. For a range of doping, the gap persists well above T c , behavior previously associated with underdoped bilayer cuprates. The anisotropy and magnitude of the normalstate gap are very similar to the superconducting state gap, indicating that the two gaps may have a common origin in a pairing interaction.[S0031-9007(97) A central issue in the physics of high-T c superconductivity is the role of coupling between the two-dimensional copper-oxygen planes in producing superconductivity. The T c of these materials tends to increase with the number of layers per unit cell. It is currently an open question whether the superconducting state order parameter symmetry will be the same in one-layer and the more strongly coupled two-layer compounds. Angle-resolved photoemission spectroscopy (ARPES) has the potential to resolve this issue since it is able to measure directly the anisotropy of the superconducting state gap (the magnitude of the order parameter). In the two-plane material Bi 2 Sr 2 -CaCu 2 O 81d (Bi2212), the gap was found by ARPES to be highly anisotropic and consistent with a d x 2 2y 2 order parameter [1,2]. We report measurements of the one-plane material Bi2201 that show a similarly large anisotropy with a smaller overall gap magnitude.In underdoped Bi2212, ARPES measurements have shown that the anisotropic gap persists well above T c [3,4], consistent with many other experiments that have shown a pseudogap or spin gap in the normal state of cuprate superconductors [5]. Current evidence for the normalstate gap in one-plane materials is much weaker than in two-plane materials, and its existence is controversial [6]. Our results show a clear normal-state gap up to high temperatures in optimally doped and underdoped Bi2201, but not in overdoped Bi2201.Single crystal samples of Bi 21x Sr 22͑x1y͒ La y CuO 61d were grown using a floating zone method, and for comparison by a self-flux method. X-ray scattering confirms that the crystals are single-phase Bi2201, and electron-probe microanalysis was used to measure the atomic ratios of the cations. Substitution of trivalent La or Bi for divalent Sr reduces the hole concentration in the CuO 2 planes. The effect of La doping goes beyond changing the carrier density [7], however, and raises the maximum T c from 10 to 30 K.A roughly parabolic dependence of T c on ͑x 1 y͒ has been observed [8]. Our optimally doped crystals (T c 29 K) come from substituting La 0.35 for Sr. With no La substitution, Bi͞Sr ratios of 2.3͞1.7 and 2.1͞1.9 give the underdoped samples with T c , 4 K and the overdoped samples with T c 8 K, respectively. The transition temperatures were taken as the zero resistance values and confirmed by SQUID magnetization measurements. The tr...