Planktonic copepods are small transparent animals swimming in water. To observe how a male finds its mate, special optical systems had to be designed. The animals are treated as phase objects and matched spatial filters allow three-dimensional recordings of the swimming behaviour in a 1-litre vessel. Application of the techniques described shows how a male cyclopoid copepod swims for 20 s in synchronicity with the female before mating. Results stemming from observations with this optical system are published in this volume.