Trials were conducted in nine commercial processing tomato fields in California from 1994 to 1995 to assess the effects of potassium fertilization on fruit yield and quality. Sites were selected to represent a range of ammonium acetate extractable soil K levels (91 to 284 mg·kg–1, top 30 cm) and K release rates (K at 1.8 to 8.5 mg·kg–1·d–1). Potassium was applied in furrow or drip irrigation during the fruit bulking stage at seasonal rates from 90 to 135 kg·ha–1. Significant yield increase (4% to 24%) was observed at three of the four sites with extractable soil K <125 mg·kg–1 (K released at <3.1 mg·kg–1·d–1); no yield response was observed at the five sites with greater soil K supply. Fruit color and soluble solids content was unaffected by K fertilization at any site. Additionally, red fruit of two cultivars (`Halley' and `Heinz 8892') were collected from 80 commercial fields in 1995 and evaluated for soluble solids content, color (of a comminuted sample as well as visual ranking of internal and external ripening disorders), and tissue K concentration. Fruit K concentration was poorly correlated with any quality characteristic. We conclude that yield response to K fertilization can be adequately predicted by either soil test method and that K supply plays a relatively minor role in tomato fruit quality under representative field conditions.