In 1994 a warm-water barnacle Solidobalanus fallax was recorded for the first time in the UK (Southward, 1995). The naupliar development of this immigrant species is now described, from larvae reared in the laboratory. The planktotrophic nauplii of S. fallax reached the cyprid stage 8 d after hatching, at 25°C. Larval development includes six naupliar and one cyprid stage, following the typical pattern of the thoracican Cirripedia. Naupliar stages have a broad rounded convex cephalic shield without dorsal and marginal spines. Nauplii have a trilobed labrum with the small teeth found in other warm-water species. The abdominal process becomes nearly equal to the dorsal thoracic spine in stage VI. The arrangement of abdominal spines and larval setation are in the usual balanoid pattern. The significant difference of the S. fallax larvae from those of the related temperate-water archaeobalanid species Hesperibalanus hesperius supports the classification of these species into separate genera.