A field experiment was conducted at the Elora Research Station in 1998 to study the effects of wild mustard competition on the growth and yield of the hard red spring wheat variety 'Celtic'. Uncontrolled natural infestation of wild mustard at 772 plant m-2 reduced light penetration, growth and yield of spring wheat. Wild mustard competition caused 44% and 51% reduction in grain yield and final biomass of spring wheat respectively. Wild mustard plants were taller than spring wheat throughout most of the growing season. In the presence of wild mustard competition, light interception by wheat was reduced, resulting in a fewer number of tillers and leaves. Wild mustard suppressed wheat height and lowered the stem elongation rate of wheat from the five leaf (Zadoks 15) to flag leaf appearance (40 DAE at Zadoks 39) growth stages. Wild mustard also influenced yield components of spring wheat. Growth analysis indicated that dry matter accumulation, crop growth rate and harvest index of spring wheat were reduced under season-long weedy conditions. Dry matter distribution of spring wheat was also affected by wild mustard. (Abstract shortened by UMI.)