Survival and growth rates of juvenile turban snail (Turbo cornutus) reared on 2 red algae, an articulated coralline alga (ACA) Marginisporum crassissima and a gelidiccan alga (Agar) Gelidium elegans, with natural and reduced diatom densities were compared to examine the relative importance of algal fronds and epiphytic diatoms as diets for the juvenile turban snail. Laboratory experiments were conducted for 8 or 9 wk with turban snail of 3 size groups, early juveniles with 1.26-mm and 2.89mm shell diameters and juveniles with an 8.1-mm shell height. The 2 diameter size groups of early juveniles grew well (22.3 ± 5.5 µm/day and 16.4 ± 4.2 µm/day, respectively; mean ± SD) on ACA with natural diatom densities, whereas the growth rates were significantly lower on ACA with reduced diatom densities in both size groups (6.7 ± 0.8 µm/day and 4.0 ± 2.2 µm/day, respectively). A significant difference in growth rate was also found between early juveniles of 1.26 mm on Agar with natural and reduced diatom densities. In contrast, no significant difference was detected in juveniles of 2.89 mm on Agar with natural and reduced diatom densities. Although turban snail juveniles of 8.1 mm reared on Agar with natural diatom densities had very high growth rates (106.1 ± 18.3 µm/day), the mean growth rate of juveniles on ACA with natural diatom densities was relatively low (10.9 ± 5.8 µm/day). The results of the 3 feeding experiments indicate that diatoms are an important food source for juvenile T. cornutus from 1.25 mm to approximately 4.0 mm, and that the importance of fronds of G. elegans as a food source increases as juveniles increase in size. In addition, the species composition and density of diatoms on ACA fronds collected from a nursery habitat of the turban snail were observed continuously for approximately 2 y. From the information gained on seasonal changes of diatom communities and experimental results, the importance of ACA turf in providing suitable diets for turban snail juveniles is discussed.