The greatest importance in the cultivation of lettuce, which is well accepted on the market, is given to healthy and strong seedlings. The productivity of the plants and the quality of the product both depend on the production of seedlings, which is therefore considered the most important stage in the entire cultivation of lettuce. The aim of this work is to monitor the dynamics of growth and development of seedlings during autumn cultivation. In addition to measuring the morphological properties of seedlings, it was also important to monitor the intensity of occurrence and determination of harmful organisms during cultivation. The experiment was set up at the experimental site of the Zagreb Faculty of Agriculture in a heated greenhouse. Two varieties of lettuce were used: Posavka and Baby leaf Coulthard RZ, and the lettuce was sown in polystyrene containers with 160 seeding places. The Posavka variety was sown during October, while the Baby leaf Coulthard RZ variety was sown during November. Since the growth and development of the seedlings both depend on the time of sowing, due to climatic factors the seedlings of the Baby leaf Coulthard RZ variety developed more slowly than the seedlings of the Posavka variety that were sown earlier. Seedlings of Posavka variety lettuce had a larger mass, while Baby leaf Coulthard RZ lettuce seedlings had more leaves. No pests were recorded during cultivation, but during December and January the relative humidity was very high, which led to the appearance of diseases such as downy mildew and gray mold. No plant protection agents were used during the cultivation of seedlings.