Chlamydomonas has long been one of the most successful unicellular organism for genetic and biochemical studies of the photosynthesis, organelle genomes and flagellar assembly. The availability of the new molecular genetic techniques is increasing interest in Chlamydomonas as a model system for research in areas like swimming behavior where it previously has not been widely exploited. The swimming behavior of Chlamydomonas reinhardtii is influenced by several different external stimuli including chemical attractants. Chemotaxis of the green alga is altered during gametic differentiation. Gametogenesis results in the conversion of chemotactically active vegetative cells into chemotactically inactive gametes. This experimental system offers the opportunity to study cellular behavior and differentiation at the molecular level with use of a wide range of molecular genetic approaches, including gene tagging by insertional mutagenesis, quantitative PCR and RNA interference. In this chapter I discuss recent progress in the field of chemotaxis in Chlamydomonas. Emphasis is placed on the signal pathways by which the two environmental cues - ammonium and light control chemotaxis and gametic differentiation.