The rheological behavior of an Arthrospira culture was studied from low to high biomass concentration. Two tubular undulating row photobioreactors (TURP-5r and TURP-10r), with a very short light path of 1.0 cm, were used during batch growth. In TURP-5r, the biomass concentration increased to 14.5 g(dw) L-1, and alterations of the physical properties and hydrodynamic behavior occurred as a result. In the past, the rheological characteristics of photosynthetic-microbe cultures were rarely investigated because of the low biomass concentration attained in the systems. Developing closed photobioreactor technologies, the optimum biomass concentration rises and the viscosity, the generalized Reynolds number (N'(Re)) and the power required for culture recycling are also subject to alteration. Starting from a biomass concentration of 4.1 g(dw) L-1, the Arthrospira culture already exhibits the characteristics of a non-Newtonian fluid. As a result of culture recycling from 2.0 to 20.5 g(dw) L-1 and an available power of 1.67 W row(-1), we demonstrated that N'(Re) is reduced from 6265 to 1148. Our experimental results showed that N'(Re) of 2345 can be reached only at a cell concentration below 11.1 g(dw) L-1, while at a cell concentration below 4.1 g(dw) L-1 N'(Re) = 4080 was reached. The power consumption (P-c) for culture recycling increased noticeably when the cell concentration rose; the highest P-c increase attained was from 2.0 to 4.1 g(dw) L-1. This is the range within which the Arthrospira culture changes from a Newtonian to a non-Newtonian fluid.