During the synthesis of fatty acids and their utilization in plastids, fatty acyl moieties are linked to acyl carrier protein (ACP). In contrast to previously cloned organ-specific ACP isoforms, we have now isolated a cDNA clone for a potentially constitutive ACP isoform from a spinach root library. Identity between the amino acid sequence encoded by this cDNA and N-terminal sequence data for ACP-II protein from spinach leaf indicates that the root cDNA encodes ACP-II. The deduced amino acid sequence for ACP-II shows 62% identity with spinach leaf ACP-I. Southern analysis suggests that multiple ACP genes or pseudogenes occur in the spinach genome. High-stringency northern blot analysis and RNase protection studies confirm that, within the region encoding the mature ACP-II, the cloned ACP sequence is expressed in leaves and seeds as well as in roots. Quantitative RNase protection data indicate that the ratio of ACP-I and ACP-II mRNA sequences in leaf is similar to the ratio of the two proteins.