Speech intelligibility was studied in a group of 25 male patients with amyotrophic lateral sclerosis (ALS). The object of the study was to determine the phonetic impairments underlying the speech intelligibility deficits that frequently accompany ALS. Analyses with a word intelligibility test indicated that the most disrupted phonetic features involved phonatory (voicing contrast) function, velopharyngeal valving, place and manner of articulation for lingual consonants, and regulation of tongue height for vowels. The mean error proportion for the five most severely affected features correlated highly (0.97) with the intelligibility score (percent correct). The phonetic feature analyses are one index of bulbar muscle impairment in amyotrophic lateral sclerosis and also may help to direct the speech management in these individuals.