The Hawaiian lava lakes offer an unparalleled opportunity to study the processes that occur during the crystallization of basaltic magma. This paper presents estimates of the rates of nucleation and growth of plagioclase in the Kilauea lava lakes, Makaopuhi and Alae, and a discussion of the processes that control the nucleation and growth. The observed growth rates perpendicular to (010) vary from 1,7 to 11.0 × 10−10 cm sec−1. The nucleation rates vary from 6.8 × 10−3 to 2.0 cm−3 sec−1. In general the rates increase with increasing crystallization at any point, decrease with increasing distance from the surface, and are higher in the shallower lake, Alae. For the most part, nucleation appears to occur heterogeneously on previously existing crystals. The growth appears to be controlled by the interface attachment kinetics and not by diffusion in the melt. The observed results are in qualitative agreement with theoretical predictions.