The effect of molybdenum concentration upon the growth of Anabaena cylindrica is studied. Within the range of molybdenum concentration that it was found possible to achieve by the methods described, the element does not affect growth on ammonium chloride, but it is required for healthy growth on nitrate or gaseous nitrogen, the optimal concentrations being about 0·075 and 0·20 p.p.m., respectively. Molybdenum appears to be unnecessary for nitrate uptake.