This study was carried out during the spring growing season of 2003 in order to determine tomato crop evapotranspiration and to develop crop coefficients under different irrigation treatments in open and closed systems. The experiment was conducted in a non-heated plastic covered greenhouse. Tomato plants were grown in perlite and irrigation was applied according to the indoor integrated solar radiation levels viz 1, 2, and 4 MJ m(-2). The results showed that evapotranspiration of tomato plants varied according to the treatments and ranged between 229 and 371 mm. The highest daily evapotranspiration values were 4.4 mm and 5.0 mm in the closed and open systems, respectively. Regarding the irrigation treatments, the highest evapotranspiration rate was obtained with the 1 MJ m(-2) treatment. Crop coefficient curves were obtained from the ratio of crop evapotranspiration (ET) to the reference evapotranspiration (ET0) estimated by pan evaporation, solar radiation and FAO Penman-Monteith equation methods on a weekly basis during growing season. The maximum crop coefficient value for tomato derived from the Penman-Monteith formula was 1.5 in the open system.