Mustard (Brassica campestris L. var. sarson Prain cv. T42) plants were grown in refined sand at graded levels of molybdenum ranging from acute deficiency (2 × 10−5 mg 1−1) to excess (2 mg 1−1). Visible deficiency symptoms appeared at 2 × 10−1 mg 1−1 or less molybdenum. Total dry matter and seed yield were maximal at 2 × 10 1−1 mg Mo 1−1. In leaves of the deficient plants there was depression in the concentrations of DNA and RNA, protein nitrogen and activities of nitrate reductase and cytochrome c oxidase but stimulation in the activities of acid phosphatase and ribonuclease and accumulation of phosphorus and nitrate nitrogen.