Field experiments were conducted in 1989 and 1990 to determine the relationship between yield loss in fibre flax and the density of volunteer barley. At volunteer densities of 50-100 plants m~^, plants which emerged 7 days before those emerging synchronously with the flax produced a significantly greater reduction in stem height (1990), stem dry weight (DW) at flowering (1989 and 1990) and seed boll DW (1990). The relationships between flax stem and boll DW and volunteer density were described at two growth stages (flowering and maturity) using a linear model. The 1990 model indicated that where volunteer tiller densities ranged from 0 to 350 m~^, interference by 90 volunteer tillers m~^ (30 plants m~2) reduced stem DW by 2-9% at the flowering stage. These losses increased to 9-18% for stem DW plus 7-18% for seed boll DW when plants were grown to seed maturity. Due to the high value of flax fibres and seed it is concluded that an economic threshold based on the control of volunteer barley can be reached at low volunteer densities (30 plants m"*^) especially if both fibre and oilseed were to be harvested.