Harvesting is one of the most important operations in microalgal biomass utilization. Two types of flocculants are widely used in flocculation. Inorganic flocculants are usually cheaper than organic flocculants, but leave more contaminants on the culture media. The combination of one inorganic and one organic flocculant and its synergy for the marine microalgae Chaetoceros gracilis harvesting process is reported. The efficiency of eight inorganic and five organic flocculants was studied. The results show that, when only FeCl3 was used, high doses were required for high levels of biomass recovery, whereas chitosan alone led to poor recoveries. However, when both flocculants were used in combined flocculation, total biomass recovery was achieved, even at low doses.