THE red algae (Rhodophyta), which like cyanobacteria have only chlorophyll a and use phycobilisomes for light-harvesting1,2, are often considered to have originated independently of other photosynthetic eukaryotes, namely the chlorophyll a/b-containing Chlorophyta and the chlorophyll a/c-containing Chromophyta3. Here we report that the red alga Porphyridium cruentum has a chlorophyll a-containing antenna complex functionally associated with photosystem I, and that polypeptides of this antenna complex are immunologically related to those of higher-plant chlorophyll a/h complexes and to those of chromophyte fucoxanthin-chlorophyll a/c antenna complexes. This establishes a clear link between organisms containing phycobilisomes and those containing chlorophyll-based light-harvesting complexes and shows that these antennae can co-exist in the same organism. Furthermore, it suggests that the light-harvesting proteins of all photosynthetic eukaryotes had a common origin and supports the idea that chloroplasts had a common ancestor4-6.