A comprehensive study of the major environmental factors affecting growth and chemical composition of Gelidium crinale is presented here for the first time. The purpose of this study was to analyze the significance of the major growth factors and to formulate them into regression models. For this purpose summer and winter experiments were conducted in small aerated seawater tanks under various light and fertilizer conditions. The effects of carbon source, pH control and microelements were also tested. Finally, an annual growth experiment was conducted in order to support the two-season experiment. The major limiting seasonal factor was found to be temperature, which positively affected weekly growth rate, dry weight and gel strength, while negatively affecting the agar percentage. Ammonium was also revealed as a major limiting factor, positively affecting chlorophyll and protein concentrations, and negatively affecting carbohydrate concentration. Other growth factors, such as light intensity, pH, carbon source and microelements, were less-or non-effective, being already almost optimal under ambient conditions.