Haematococcus Pluvialis is a type of freshwater green unicellular microalgae that can be used as economical natural source of astaxanthin. To guarantee optimal algae production and the proper conditions for the obtainment of high yield of astaxanthin, the growth process should be carefully monitored and controlled. When on-line measurements are not available, state estimators can provide a useful tool for process monitoring. In this paper a geometric observer is designed based on an extended version of the Droop model for microalgae growth, identified on the basis of batch experiments. The observer functioning and convergence are tested using experimental data obtained from a laboratory scale bioreactor.