The photosynthetic performance of a conical, helical tubular photobioreactor (HTP) incorporating Chlorella sorokiniana was investigated under conditions of high temperature and light intensity during midsummer in an outdoor environment. Although the culture medium temperature exceeded 40degreesC for approximately 5 h each day, peaking at 47.5degreesC under sunny, conditions, a photosynthetic productivity of 30.0 g.m(-2) (installation area) . day(-1) and a photosynthetic efficiency of 8.66% [photosynthetically active radiation (PAR), 400-700 nm] were achieved. A maximum photosynthetic productivity of 33.2 g . m(-2) . day(-1) was achieved on a sunny day, when solar energy input was also maximal (11.5 Mj . m(-2) . day(-1) [PAR]). On the other hand, a maximum photosynthetic efficiency of 9.54% was obtained on a day that was rainy in the morning and cloudy in the afternoon, and there was relatively little solar energy input. The average daily photosynthetic efficiency over the two culture periods (August 4 to 7 and August 10 to 13, 1999) was 7.25%. Thus, a high level of photosynthetic performance was achieved in the conical HTP incorporating Chlorella sorokiniana despite the fact that culture medium temperature was not controlled. The use of Chlorella sorokiniana in the conical HTP should be a good choice to produce microalgal biomass during the summer under field conditions.