The objectives of this preliminary study were to optimize a carbon dioxide (CO2) application system for field-grown raspberry (Rubus ideaus) under high tunnels and then to compare plant photosynthesis, growth, and fruit yield with and without CO2. Based on plant photosynthesis measures before CO2 application, we placed the drip irrigation tape to apply CO2 in the middle of the plant canopy at 100 cm aboveground and split daily CO2 application from 0700 to 1100 hr and 1400 to 1800 hr. In the morning hours, CO2 concentration in the tunnel was 18% higher than in the afternoon; wind speed often increased later in the day, which may have moved the CO2 even in the tunnel. We maintained an average CO2 concentration of 436 ppm for 4 months, applying about 25 tons of CO2. In tunnels enriched with CO2, yield and berry size from plots 20 ft in length increased 12% and 5%, respectively, compared with untreated tunnels. We measured no corresponding differential response in the number of 6-oz baskets harvested from the full tunnel, leaf CO2 assimilation, stomatal conductance (g S), or fluorescence, raspberry cane height, pruned cane biomass, cane diameter, or carbohydrate content of the fruit after CO2 application. Documenting potential plant physiological changes should therefore be a focus of future research.