Technical Abstract: Two cultivars of fresh market tomato (‘Oregon Spring’ and ‘Red Sun’) were each grown in two high tunnels at Beltsville, MD covered with a contrasting material of similar thickness (0.152 mm) and durability (4-year polyethylene). One covering material (Tyco Tufflite IV) transmitted ambient solar UV radiation from 290 to 400 nm (designated +UV) while the other material (Dura-Film Super 4) blocked UV wavelengths below 380 nm. (designated -UV). Both films transmitted comparable amounts of photosynthetically active radiation (PAR) from 400 to 700 nm. Ripe tomato fruits comparable in size and development were collected at maturity from plants of the two cultivars grown in each high tunnel under the contrasting covering materials. Four lots of tomatoes of each cultivar and each UV treatment were assayed for total phenolic (TP) content by a colorimetric Folin-Ciocalteu (FC) assay and for content of individual phenolic acids by a high performance liquid chromatography (HPLC)-diode array detection (DAD) procedure. Three phenolic acids were extracted in the base hydrolyzed fraction; these were identified as caffeic acid, p-coumaric acid, and ferulic acid. Caffeic acid was the predominant phenolic acid in both ‘Oregon Spring’ and ‘Red Sun’ tomatoes grown in the two high tunnels. The total concentration of these three phenolic acids measured by HPLC was approximately 20% higher under +UV than under -UV treatment; this was true for both cultivars. Similar differences were obtained in the concentration of total phenolics in tomato fruits harvested from plants grown in the two contrasting high tunnels when the extracts were assayed using a FC assay. These results indicate that the phenolic content of tomato fruits is significantly impacted by the amount of ambient solar UV radiation available. Since phenolic compounds are known to play a key role as antioxidants in human nutrition, subtle differences in phenolic composition between the two high tunnels as a result of differences in the UV transmission properties of these different covering materials may be of considerable importance.