Crystal growth and associated cytoplasmic changes were observed in Halimeda monile and H. incrassata by electron microscopy. Aragonite crystals were first formed at the filament wall surface in spaces between the utricles of segments about 36 hours old. As the segments aged, the interutricular spaces became filled with crystals. No organic matrix was observed at the sites of crystallization. The crystals exhibited random orientation. Two unidentified cytoplasmic components, one of fibrillar spherical form and another of high electron density, were present in large numbers.