AbstractA 20‐kD protein identified as a subunit of the iron‐binding protein ferritin is present in S.purpuratusandL. pictussea urchin embryos. The synthesis of the protein is stimulated by an elevation in temperature or by an increase in iron supply. The developmental expression of this protein and its regulation during normal development and upon heat shock was investigated. InL. pictus, ferritin is present in the unfertilized egg and, as determined by Western blot analysis, its concentration remains approximately constant after fertilization up to the gastrulc‐pluteus stage; there is a small transient decrease in the level of the protein in the early blastula at a time coinciding with the first clear indication of its de novo synthesis. Northern blots reveal no cytoplasmic ferritin transcripts in the unfertilized egg, but there occurs a dramatic increase in the RNA level from the late morulaearly blastula stage (12–14 hr) to the mesenchyme blastula‐early gastrula (25–30 hr) stage. This developmentally regulated increase in the constitutive concentration of ferritin RNA is correlatable with the normal onset of synthesis of the protein. The overall degree and nature of induction of ferritin by heat is dependent on the developmental stage: at 10–16 hr postfertilization heat shock elicits an increase in both the concentration of RNA and the synthesis of the protein; in hatched blastula (18 hr) and in later embryos heat shock increases ferritin synthesis, without a corresponding increase in the mRNA level. It appears that different mechanisms operate in the developing sea urchin embryo to regulate the expression of ferritin during normal development and on exposure to heat stress, one dependent on the concentration of ferritin transcripts and another operating at the level of translational control. © 1993W