A photobioreactor in the form of a 245-m-long loop made of plexiglass tubes having an inner diameter of 2.6 cm was designed and constructed for outdoor culture of Spirulina. The loop was arranged in two planes, with 15 8-m-long tubes in each plane. In the upper plane, the tubes were placed in the vacant space between the ones of the lower plane. The culture recycle was performed either with two airlifts, one per plane, or with two peristaltic pumps. The power required for water recycle in the tubular photobioreactor, with a Reynolds number of 4000, was 3.93 × 10−2 W m−2. The photobioreactor contained 145 L of culture and covered an overall area of 7.8 m2. The photobioreactor operation was computer controlled. Viscosity measurements performed on Spirulina cultures having different biomass concentrations showed non-Newtonian behavior displaying decreasing viscosity with an increasing shear rate. The performance of the two-plane photobioreactor was tested under the climatic conditions of central Italy (latitude 43.8° N, longitude 11.3° E). A biomass concentration of 3.5 g L−1 was found to be adequate for outdoor culture of Spirulina. With a biomass concentration of 6.3 g L−1, the biomass output rate significantly decreased. The net biomass output rate reached a mean value of 27.8 g m−2 d−1 in July; this corresponded to a net photosynthetic efficiency of 6.6% (based on visible irradiance). © 1993 John Wiley & Sons, Inc.