A study examined sentence production errors and syntactic complexity in students' writing in two modes of discourse and at three grade levels. Subjects, average and high developmental students enrolled in seventh, ninth, and eleventh grade classes, each wrote two compositions, one in the descriptive the other in the persuasive mode. Data analysis consisted of two parts, first a study of the relationship among syntactic maturity, grade level, sentence error rate, and developmental level within grade level; then a descriptive error analysis of two types of sentence production errors--sentence fragments and run-on sentences. Among the many results of the analysis were the following: (1) a positive relationship existed between composition length and age and student ability; (2) mode of discourse had an effect on syntactic complexity and length of composition, as well as on the number of run-on sentences; (3) the oldest students and students in higher within-grade groups made fewer sentence errors; (4) developmental differences between high and average groups were present in error rates and length of compositions; and (5) the relationship between a sentence fragment and an accompanying sentence was usually between the fragment and a preceding sentence. (FL)