Diatoms are unicellular algae, which due to their importance for the global primary production and their cellular and genetic complexity, became popular subjects of physiological and molecular biological research in the recent years. The increasing genomic information gathered on diatoms since the last decade promotes diverse analyses of their steady state RNA levels, which are commonly performed via quantitative real-time PCR (qPCR), a technique which excels in sensitivity and dynamic range. Up to now there are only a few studies on suitable endogenous reference genes in diatoms. Such reference genes are crucial for any relative qPCR study and must feature stable transcript levels between all the investigated experimental conditions. Therefore we expanded the data on suitable endogenous reference genes by thorough testing of ten potential genes in the model diatom Phaeodactylum tricornutum at light and time discriminate conditions. Stably expressed genes for these conditions will be of great use for any diatom study dealing with time and light dependent effects. Samples of algae grown in a 16 hours low light photoperiod and dark transitioned cells were investigated over a period of up to 33 hours. A set of three endogenous reference genes was found to be stably expressed in a light and time independent manner: the TATA box binding protein TBP, the ribosomal protein S1 RPS and the hypoxanthine-guanine phosphoribosyltransferase HPRT. Other commonly used reference genes like actin, histone H4 or 18S ribosomal ribonucleic acid did not perform well and are thus unsuited for expression analysis in light or time dependent experimental setups.