Emiliania huxleyi is a cosmopolitan haptophyte that significantly contributes to global carbon cycling. Frequently occurring massive blooms of this algae in the oceans can be terminated by viruses. An expressed sequence tag (EST) approach was used to analyse the infection of E. huxleyi strain CCMP1516 infected with the virus EhV86. cDNA libraries were constructed before, and 6, 12, and 24 hours after infection, respectively, with the virus and 1000 1400 clones from each library were randomly selected and annotated with PartiGene and PhyloGena. Viral genes were identified using BLAST-Search of the known viral genome. From the nature and abundance of sequences in the library we could deduce the partial transcriptome of both, host and virus, through the infection process and draw conclusions on the infection mechanism. 6 hours after infection the algal transcriptome changed significantly although only few viral transcripts were present. At this point, changes mainly concerned transcripts related to photosynthesis and protein metabolism. However, after 24 hours viral transcripts were most abundant. Viral transcripts found encode proteins involved in protein degradation, nucleic acid degradation, transcription and replication.As a next step we are now conducting a DNA microarray-based genome comparison of various E. huxleyi strains with a 10000-EST-microarray. Both DNA and RNA from different E. huxleyi strains will be hybridized to the array with the aim to (1) detect genomic differences in virus-resistant and susceptible strains and (2) to analyse in detail gene expression patterns of the different strains after infection with EhV86.