text: Dexter's nemesis is rival classmate Mandark Astronomonov (voiced by Eddie Deezen). Like Dexter, Mandark is a boy genius with his own laboratory, but his schemes are generally evil and designed to gain power or downplay or destroy Dexter's accomplishments. In revival seasons, Mandark becomes significantly more evil, becoming Dexter's enemy rather than his rival, and Mandark's laboratory changes from brightly lit with rounded features to gothic-looking, industrial, and angular. Mandark's unrequited love for Dee Dee is shown as a pivotal weakness, notably near the end of the Ego Trip television film.
  Recurring segments
  Every Dexter's Laboratory episode, with the exception of "Last But Not Beast", is divided into different stories or segments, each being 7–12 minutes long. Occasionally, a segment centers on characters other than Dexter and his family. Two segments are shown primarily during season one: Dial M for Monkey and The Justice Friends. Dial M for Monkey is the middle segment for six episodes of season one, and The Justice Friends takes its place until season one's end. After the first season, characters from extra segments were not as frequently seen, with one exception being the  Dial M for Monkey segment, "Peltra", which aired in season two.