SummaryA study was made of the caryopsis weights ofAvena fatuagrown in the field and in pots and of the competitive ability ofA. fatuafrom heavy seed (≥18 mg) and light reed (≤11 mg) grown at a depth of either 25 or 75 mm with spring barley sown at 25 mm.The weight of field‐ and pot‐grown caryopses ofA. fatuatypes fA, fB and fC ranged from 3 to 25, 5 to 23, and 5 to 25 mg respectively. With one exception, both the field‐ and pot‐grown samples showed a double normal distribution pattern. The mean weights of pot‐grown primary and secondary caryopses of fA, fB and fC were 15 and 9, 19 and 11, and 18 and 11 mg respectively.In a competition experiment,A. fatuagrown from heavy seeds sown at 75 mm depth with an equal number of barley produced 47% more panicles, 54% more seed and 56% more dry weight per plant than plants from light seeds. When barley and both seed weights ofA. fatuawere sown at 25 mm depth these differences were smaller, being 21, 28 and 34% respectively.When the barley was at Zadoks stage 87, and when the barley andA. fatuawere grown at equal densities at 25 mm depth, the dry weight of barley was reduced from 10·4 to 7·7, and to 5·8 g per plant by light‐ and heavy‐seededA. fatua, respectively. When the barley was grown at 25 mm and theA. fatuaat 75 mm, the weight per barley plant was reduced to 9·5 and 7·2 g byA. fatuaplants grown from light and heavy seeds respectively. Reductions in numbers of grain were caused mainly by a reduction in the n