Data were collected on the origin, frequency, morphology, reproductive behaviour, and cytology of spontaneously occurring autotetraploids in pearl millet, Pennisetum americanum (L.) K. Schum. Autotetraploids occurred at frequencies as high as 1 per 13 and 292 plants in poly-embryonic and non polyembryonic inbred 'Tift 239' material, respectively. Tetraploids could not be visually distinguished from diploids at the vegetative stage. However, at anthesis the tetraploids, in contrast to the diploids, has poor pollen shed and the inflorescence had a tufted appearance due to longer and more frequent stigmatic hairs on the pistil. Tetraploids exhibited less stainable pollen, greater variability in pollen size, and larger guard cell areas than the diploids. Seed set of selfed tetraploids was 1% that of selfed diploids. The mean numbers of chromosome configurations observed per microsporocyte at metaphase I were 2.64 univalents, 8.97 bivalents, 0.38 trivalents, and 1.49 quadrivalents. Polyembryony in Tift 239 pearl millet is apparently associated with tetraploidy. Lines giving high numbers of polyembryonic caryopses also give high numbers of tetraploids.