Diquat treatments were applied to flax (Linum usitatissimum L.) over a 5-yr period to determine its effect on some agronomic and quality characteristics when applied at 0.28 and 0.56 kg a.i./ha at the 75, 50, 25 and 0% green capsule stages. In most cases the spray applications effectively hastened drying of the plants, eliminating the need for windrowing. Desiccation of capsules and leaves was generally rapid and complete, but stem desiccation was variable: in some cases the plants were dried completely but in a few cases only the top 20% or less was dried. The 0.56 kg a.i./ha rate gave a more rapid and more complete desiccation than the 0.28 kg rate. In most cases, seed yields and 1000-seed weights were reduced by applications made at the 75 and 50% green capsule stages, and sometimes at the 25% green stage, particularly when maturation proceeded slowly. In some instances germination percentages were reduced, particularly by the earliest applications. Effects on oil content, iodine value and volume weight were usually minimal. Harvesting of the sprayed crop soon after it was dry was necessary to avoid the risk of accelerated seed weathering. It was concluded that delaying diquat application until the 25% green capsule stage, or later, was necessary to assure minimal effect on seed yield and quality.