An experimental helical-tubular photobioreactor has been designed for controlled, continuous production of Nannochloropsis sp. Its main advantages are: (1) combination of large ratio of culture volume to surface area along with the optimised light penetration depth, (2) easy control of temperature and contaminants, (3) effective spatial distribution of fresh air and CO(2), (4) better CO(2) transfer through extensive interface surface between fresh air and culture-liquid medium and (5) novel automated flow-through sensor providing continuous cell concentration monitoring. Nannochloropsis sp. population density reached maximum value under rather high temperatures and combined natural and artificial light conditions. An average daily increase of 30 x 10(6) cells ml(-1) was obtained at population densities above 350 x 10(6) cells ml(-1) allowing daily harvesting rates of at least 10% the total volume. Measured cellular density productivity data and estimated volumetric productivity range of 1.10-3.03 g l(-1) day(-1), are among the highest Nannochloropsis sp. productivities reported in the literature. (C) 2010 Elsevier Ltd. All rights reserved.