In the experiment conducted in 2004-2006 there was evaluated the effects of living mulches in tomato (Lycopersicon esculentum Mill.) cultivation. As the living mulches there were used white clover (Trifolium repens L.), serradella (Ornithopus sativus Brot.), perennial ryegrass (Lolium perenne L.), and pot marigold (Calendula officinalis L.). All these species were sown in the field in early and late April, at the time of tomato planting (20 May) or three weeks thereafter. In some treatments growth rate of living mulches was restricted by mowing or gliphosate application. Tomato plants cv. Delfine F1 were trained to one stem and tied to stakes. Harvest of fruits was conducted in one week intervals since the end of July till the end of September. In samples of fruits harvested at the end of August there were evaluated the contents of dry matter, vitamin C, total and reducing sugars. Data of the study showed that serradella and perennial ryegrass were less competitive for tomato than white clover and marigold, due to lower production of biomass at the end of growing period. In the average they reduced the yield of fruits by 21.6%. Irrespective of the plant species used as living mulch, the delay of undersowing to the date of tomato planting or three weeks thereafter as well as restriction of their growth by mowing were the efficient methods for increasing tomato fruit yield. Chemical analysis of tomato fruits did not show any significant influence of compared living mulch species or date of undersowing on contents of dry matter, vitamin C, total and reducing sugars.