We studied fasciculation potentials in amyotrophic lateral sclerosis (ALS), and in other neurogenic disorders, in strength and single-fiber electromyography-matched muscles. Benign fasciculations were studied in 3 normal subjects. Fasciculations were more stable and easier to recruit voluntarily in the early phase of ALS; later, fasciculations were more unstable, more complex, and less likely to be voluntarily recruited. Stable fasciculations, representing part of large, reinnervated motor units, and probably arising from distal axonal sprouts, usually had a higher firing rate than more complex and unstable fasciculations. In weak muscles in ALS, fasciculations were generally more unstable than in other neurogenic disorders. In normal-strength muscles fasciculations are less complex in ALS than in other disorders. Benign fasciculations have a high firing rate and normal morphologic parameters. In ALS fasciculations arise proximally early in the disease and distally in the later stages. Fasciculation may be important in excitotoxic cell death in ALS.