The development periods and areas of the leaf mines by larvae Liriomyza trifolii (Burgess) were studied in the experimental greenhouse. The mean development periods of the mines was 3.4 days (58.9 DD), and the mean area of individual mines was $0.6 extrm{cm}^2$. Incidence of the foliar leaf mines was examined three times in the commercial tomato greenhouse. The mine numbers in each tomato leaflet varied from 1 to 9, but the leaflets with 2 mines were the most frequent case. The area of individual mine remained relatively constant regardless of the increase of mine numbers per leaflet (0.37 - $0.41 extrm{cm}^2$). There was a significant relationship between the total mine number and total leaf area mined ($ extrm{r}^2$=0.72). Counting the total mines provides the baseline information for the decision making and assessing yield loss in the tomato greenhouses.