Cerro Negro volcano in Nicaragua first erupted in 1850. It has erupted at least ten more times, the last in February 1971. It is therefore a polygenetic, parasitic cinder cone that may eventually evolve into a composite cone. Lavas produced during the five important eruptions since 1923 have been sampled. These recent lavas from Cerro Negro are unusually phenocryst-rich basalts (29%–43% phenocrysts by volume) with phenocrysts of plagioclase (An96−85), olivine (Fo81−72), clinopyroxene (En45 Wo38 Fs16 - En41 Wo45 Fs14), and subordinate titanian magnetites (Usp16–20). All of the basalts have relatively high Al2O3 and low K2O and other incompatible element contents. There have been gradual but significant changes in lava composition with time. All multiply sampled flows show significant intra-lava inhomogeneity. The Fe-Mg contents of analyzed olivines indicate disequilibrium between the olivines and host lavas. All of the inter-lava compositional variation, most of the mineral-lava disequilibrium, and most of the intra-lava inhomogeneity are the result of differential olivine and clinopyroxene accumulation during magma ascent. The magma originated from an essentially homogeneous magma chamber. Phenocrysts in the basalts could have experienced much of their growth during ascent. Ascent velocities were probably on the order of 101–10−5 cm/sec. Increasing efficiency of crystal settling with time suggests that a narrow feeder, probably a north-south oriented dike, may be widening beneath the cone.