Mucolipidosis Type IV (ML IV) is an autosomal recessive genetic disorder characterized by severe psychomotor impairments and ophthalmologic abnormalities. Reports on the cognitive development of people with ML IV are limited, but suggest that achievement of language and cognitive milestones varies between a 3- and 18-month level. There is also variability in reports of whether people with ML IV make developmental progress, regress, or remain static after infancy. This study examines the longitudinal development of a young child with ML IV who participated in an augmentative and alternative communication (AAC) intervention.At 26 months, the child displayed significant developmental delays on direct assessment, with the exception of a possible relative strength in receptive language. An examination of adaptive behavior over time indicated improvements in raw scores but declines in standard scores across all domains. She learned to use 13 new words with a speech generating device (SGD) by the end of intervention.These results add to literature on the clinical manifestations of ML IV and indicate that although children with this disorder have deficits in many domains, they may be most severely affected in gross motor or oral motor development and have a relative strength in receptive language. Moreover, this child made progress in all domains of adaptive functioning, but at a slower pace than typically developing children. She also gained expressive vocabulary in the AAC intervention, and this may have supported her development in other areas.