Plant cells are known to elongate exogenously provided fatty acid (FA), but the subcellular sites and mechanisms for this process are not currently understood. When Arabidopsis leaves were incubated with 14C-FAs with <or=16 carbons, the label appeared in elongated and desaturated products. Laurate elongation was 85% inhibited by 50 microm cerulenin, an inhibitor of ketoacyl-acyl carrier protein (ACP) synthetase I/II. In contrast, haloxyfop, an inhibitor of cytosolic acetyl-coenzyme A (CoA) carboxylase, inhibited only elongation into very long chain FAs (>or=20 carbons) but not synthesis of 14C-unsaturated 18-carbon or 16-carbon FAs. Isolated pea chloroplasts were also able to elongate 14C-FAs (<or=16 carbons) in the light. No detectable 14C-acyl-CoA intermediates were formed during 14C-laurate elongation, whereas 14C-acyl-ACP accumulated to 2.3 microm. These data indicate that the elongation of exogenous medium-chain FAs to 16- and 18-carbon FAs occurs primarily in the chloroplasts, most likely via the enzymes of de novo FA synthesis. An Arabidopsis mutant with a T-DNA insertion in At4g14070 (AAE15) was reduced 80% in 14C-laurate elongation into 16- and 18-carbon FAs. AAE15 has sequence similarity to long-chain acyl-CoA synthetases and a predicted N-terminal plastidial targeting sequence. Direct acyl-ACP-forming activity from FA and ACP was observed in extracts of Arabidopsis leaves and isolated chloroplasts but aae15 plants had markedly reduced in vitro acyl-ACP synthesis activity. Together these results demonstrate that plants possess a mechanism for direct activation of FA to ACP in the plastid via an acyl-ACP synthetase encoded by At4g14070.