Greenhouse experiments were conducted for two consecutive seasons to investigate the influence of volunteer durum wheat cultivars and density on lentil growth and yield. It is speculated that interference may be severe whenever wheat and lentils are rotated in semi-arid regions. Lentil:durum wheat ratios of 1:0, 1:1, 1:2, 1:4 and 1:6 were evaluated utilizing two durum wheat cultivars, Hourani and ACSAD 65. The results indicated that wheat interference did not influence lentil growth for the first 80 days after crop emergence, but afterwards, the lentil height, straw and seed yield were affected by the presence of wheat. A 50% reduction in either lentil straw or seed yield was estimated from the interference of a single plant per pot. The semi-tall cultivar Hourani had more adverse effects on lentil growth and yield than the semi-dwarf cultivar ACSAD 65.