Surface resistance measurements of films of YBa2Cu3O7deposited onto single‐crystal substrates of LaGaO3and SrTiO3have been made at a frequency of 22 GHz. The measurements were made in either a copper or niobium cavity by replacing the end wall with the superconducting film. Typical surface resistance at 20 K are 1–2 m&OHgr; for films on LaGaO3and 6–8 m&OHgr; for films on SrTiO3, as measured in the copper cavity. The LaGaO3values lie within the sensitivity range of the Cu cavity (∼2 m&OHgr;) and can only be considered upper limits. Similar measurements in a Nb superconducting cavity resulted in a surface resistance value of 0.2±0.1 m&OHgr; at 4 K for the best LaGaO3‐based film. This value is more than an order of magnitude lower than Cu, and suggests that LaGaO3‐based films may offer immediate advantages in a number of applications.