The potential of petroleum sprays to thin navel orange ( Citrus sinensis ) crops in the San Joaquin Valley of California was examined in 1996, 1997 and 1998. Petroleum oils had not been used within the experimental site as adjuvants in other sprays or as pesticides in the previous year or during the experiment. `Bonanza' navel oranges trees were treated annually or in alternate years with a light narrow-range petroleum oil [distillation midpoint of 415 °F (213 °C)], a medium narrow-range oil [distillation midpoint of 440 °F (227 °C)] and/or heavier oil [distillation midpoint 470 °F (247 °C)] in a range of applications from 5 to 15% by volume in a total spray volume of 200 gal/acre (1870 L·ha -1 ). Trees treated with oil in 1996, 1997 and 1998 had 38% and 27% fewer fruit per tree in 1997 and 1998, respectively compared to trees not treated with oil indicating that crop thinning had occurred. In 1998, yield was lower in the trees that had been treated with oil annually for three consecutive years. Consecutive, annual applications of petroleum oil applied 1 to 3 weeks after petal fall produced a shift from smaller fruit sizes to larger fruit sizes beginning the second year.