Plastids of diatoms are surrounded by four membranes. The outermost membrane is continuous with the endoplasmic reticulum and therefore is termed chloroplast ER (CER) membrane. The complex ultra structure of diatom plastids naturally requires more transport steps to import nucleus encoded proteins into the plastid compared to higher plant plastids which possess only two envelope membranes. Several hypothetic models for the import of pre-proteins into the complex plastids of diatoms are discussed. Common to all these models is the postulation of a first co-translational transport step into the chloroplast endoplasmic reticulum lumen via the Sec61 translocon. Furthermore, all models postulate transport via a translocator in the innermost mem-brane similar to the Tic complex (translocon of the inner chloroplasts envelope) of higher plant plastids. The models differ, however, with respect to their explanation of transport out of the CER-lumen and into the inter-envelope space: either translocators, vesicles crossing the periplastidic space or putative membrane channels connecting CER-lumen and the inter-envelope space have been proposed. To investigate the presence of such a hypothetic connection between the CER-lumen and the inter-envelope space, we expressed different pre-proteins in the diatom Phaeodactylum tricornutum that were fused to self-assembling fragments of GFP (GFP1-10 and GFP11). Complementary fragments were fused to marker proteins of the CER-lumen and the inter-envelope space, respectively. Our data indicate that the GFP1-10 and GFP11 fusion proteins are located in two separate compartments which are not connected to each other.