This is an investigation of the phonological units used by preschool children. Twenty-four English-speaking children aged 4;0 to 5;0 were given three experimental tasks which investigated their ability to segment initial consonant clusters into phoneme-length units: (1) in a segmentation task they gave the first sound of initial cluster words; (2) in a grouping task they made initial cluster words a separate category or a subset of a larger category of words beginning with the same sound; and (3) in a symbolization task they used colored blocks to represent the sounds in initial clusters. The children were aware that words are composed of smaller segments and treated the segments as discrete units before they could identify the number of segments in a word. Data support the hypothesis that children treat clusters as units before they segment clusters into component singletons. The ability to treat the cluster as composed of two parts and to relate these parts to singleton sounds correlated with the children's prereading knowledge. (Author)