This study provides a qualitative and quantitative description of larval development ofTetrapygus nigerto metamorphosis. After fertilization, the length of the planktonic interval (developmental time) forT. nigerat room temperature (14–18°C) ranged from 94 to 120 days. Competent larvae were induced to metamorphose by transferring them into culture vessels containing bacterial films and diatoms. About 40% of the larvae reached the juvenile stage, with complete metamorphosis from the feeding larval stage to the feeding adult stage taking 4–6 days. Although the morphology ofT. nigerlarvae is atypical compared with most regular extant echinoid species, it is similar to another arbacioid species,Arbacia punctulata.This suggests that larval morphology needs to be included in studies aimed at establishing phylogenetic relationships. The large larval size (>3.0 mm close to metamorphosis) and uniform pattern of ciliation observed inT. nigerlarvae have also been observed in non-feeding larvae. However, asT. nigerlarvae feed, we believe these characteristics reflect a functional solution to the swimming and feeding requirements of a long planktonic life. The maximum size (>4.5 mm) achieved byT. nigerlarvae in thisin vitrostudy represents the largest recorded for echinoid larvae.