"Specialization" refers to the tendency of offenders to repeat the same offense type as offending continues. The current study adds to the investigation of how juvenile delinquent careers unfold over the first 20 offenses; the evidence was examined to determine whether there is any support for criminal specialization; and, if so, whether this occurred for all or only some types of offenses. Data were collected and categorized by a police agency in a large Canadian city. The original sample consisted of the criminal histories of 4,565 youths up to the sampling date of December 31, 1991. Included in this sampling frame were 200 serious habitual offenders (SHO's) and 4,365 youths who were not yet designated as serious habitual offenders. A disproportionate random sampling procedure was used to ensure a sufficient number of cases for the analysis. There were five general classification categories of criminal acts: violence, burglary, theft, other property offenses, and delinquency. The findings indicate that some delinquent acts were more prevalent than others, suggesting the possibility of specialization. SHO's appeared to be well entrenched in a delinquent lifestyle, as indicated by 3,050 police contacts in their first 20 transition points. Violent offenses were the least common of all offense categories. For an overview of the propensity to specialize across offenses, the average Forward Specialization Coefficient (a measure of specialization that corrects to some extent for the frequency of the offense) was calculated for the first 20 offense transitions. The general trend across the first 20 transitions suggests that the level of specialization was not common across all types of crime. As delinquent careers unfolded, the least specialized categories of offenses were "theft" and "violence." Youths had a tendency to specialize in "other property" offenses before the 13th transition, indicating that youths had a tendency to specialize in this offense type in the earlier stages of delinquent careers. Specialization in burglary was apparently more pronounced than specialization in other offense types. Overall, the findings are consistent with conceptual arguments regarding social behavior as complex systems prone to episodes of upheaval, collapse, and later regeneration of similar patterns of behavior at both the micro and macro levels. 4 tables and 52 references