The development of sentence coordination in children using sentences conjoined by "and" was studied to test the adequacy of the transformationally based derivational theory of complexity. Two cross-sectional experiments were conducted using 18 sentence types with children between the ages of three and five. One experiment used an elicited imitation procedure, and the other used an act-out comprehension procedure. It was found that sentential coordinations were no easier than phrasal coordinations and that forward forms were not easier than backward forms. Predictions based on the derivational theory of complexity were not supported. A second analysis was carried out on the data from the imitation experiment. There were significantly more elaborations and reduction responses in the four-year-olds than in either of the two age groups. Spontaneous speech protocols from a longitudinal study of three children were also analyzed, and the data show that the earlier forms of coordination to appear in children are phrasal forms. The forward forms have a primacy over the backward forms in both the spontaneous speech and elicited production data, but not in the comprehension and imitation data. This asymmetry may be due to a planning difficulty for the backward forms. It is concluded that the ontogenesis of coordination is not well described by the derivational theory of complexity. (SW)