Organic farming seems to be a solution in environmentally sensitive zones such as the protection zone of Tahtalı Dam, which supplies fresh water to Izmir, the third largest city in Turkey. Greenhouse vegetable production is the major agricultural activity in this area (Menderes Township). However, due to the pollution risk from agriculture, governmental authorities have issued a regulation discouraging the use of chemicals within the collection basin of the dam. An on-farm project was conducted in 2001 to introduce organic greenhouse vegetable production in the Tahtalı Dam preservation area and to serve as a model for other areas in Turkey facing similar ecological problems. Two irrigation levels and four organic fertilizers were tested on the growth of cucumbers (Cucumis sativus cv. Sardes). The four fertilizers were two rates of farmyard and poultry manure (30 and 50 tons ha−1) with and without two organic fertilizers based on either bacteria or algae. During the growing period, pests and diseases were monitored weekly and preparations allowed in organic agriculture were used when necessary. Plant diseases did not create any significant problem, but the pest population, in particular spidermites, increased. No significant differences in cucumber yield were found between the fertilizer and irrigation treatments tested. However, it should be noted that yields from the addition of 30 tons ha−1 of farmyard or poultry manure and organic fertilizers were the same as those for 50 tons ha−1 of manure alone. The average total yield was 13 kg m−2, within an acceptable range for short-term crop production in this region, but 55% lower than a conventional substrate culture trial nearby. However, due to reduced production costs and a price premium for organic products, the organic produce yielded a net return of US$1.12–1.79 per m2 whereas that for conventional bag culture produce was US$0.55 per m2. Our results indicate that, under present conditions, organic greenhouse cucumber production has less environmental impact and is more economic than conventional bag culture, due to reduced chemical input and reduced operating and initial investment costs.