This article investigated the relationship between age at onset of canonical babbling and audibility of amplified speech in children with hearing impairment. Thirteen children with severe-profound hearing impairment and two children with normal hearing participated in a longitudinal investigation of vocalization development. A nonconcurrent multiple baseline design was used to analyze vocalization recordings obtained during two phases (hearing aid [HA] and cochlear implant [CI]). Audibility during HA and CI use was calculated using the Speech Intelligibility Index (SII). Earlier ages of canonical babble onset were related to greater audibility of the speech signal during HA use. Children who developed canonical babble had an SII of 0.35 or greater. SII was a statistically significant predictor of age of onset of canonical babble. Results support the concept of an "essential" level of audibility for onset of canonical babble. Findings are discussed relative to their methodological and clinical implications regarding treatment decision making.