This thesis describes the cytochrome p450 superfamily in the pennate diatom Seminavis robusta. This diatom has a benthic way of life and can adhere to surfaces, and is a new candidate model organism representing the pennate diatoms. The genome of S. robusta is currently being sequenced and is characterized. This revealed that the genome of S. robusta contains 68 genes encoding CYPs, an unusually high number compared to other related organisms. Several unique families and subfamilies were discovered, but also known families from both plants and animals. This reflects the diversity of factors that may have contributed to the evolution of these genes in diatoms, and possibly how a benthic lifestyle may drive this process.Both bioinformatical tools and transcriptional analysis have been used to characterize and explore this superfamily. An experimental study was performed by cultivating S. robusta at different conditions, including normal temperature and light, lower temperature and a day/night cycle. This confirmed some of the CYPs’ involvement in light-/dark-responses, as well as some responses that seemed coupled to temperature.As part of the study, a comparison was made between two software programs used for analyzing gene expression data. This concluded that replacing the software currently used with a new software program with more possibilities will be beneficial for ensuring a higher quality assurance when publishing RT-qPCR data, as well as one step closer publishing according to the MIQE-guidelines.