Summary 1. The Ponto‐Caspian invader, Hemimysis anomala, was recently found in large numbers in a gravel‐pit lake connected to the Lower Rhine. Mysids were sampled with Perspex traps between September 2002 and April 2003 to study the population dynamics, vertical migration and feeding. 2. The abundance (as catch per unit effort, CPUE) of H. anomala declined from 270 individuals (ind.) trap−1 (4 h)−1 in December to below 4 ind. trap−1 (4 h)−1 in April. Average lengths ranged from 4.9 mm in autumn to 9.9 mm in March. The length–weight relationship of virgin females changed throughout the sampling period, from a size‐corrected wet weight of 5.1 mg in September to 16.6 mg in April for a female of 7.8 mm. 3. Successive monthly samples taken over 24 h revealed that H. anomala preferred the surface at twilight and night. During dawn the mysids migrated to the middle and bottom layers and were hardly found during day. Diel vertical migration clearly depends on the proximate factor light. 4. Stomach analyses revealed that larger H. anomala preferred zooplankton, whereas small individuals fed more on phytoplankton. The seasonal comparison showed an increasing percentage of zooplankton with increasing length. The proportion of zooplankton in the stomachs of large H. anomala individuals was highest during night and lowest during day. 5. The results suggest that H. anomala may become an important link between primary/secondary production and higher trophic levels in the food web of its new environments at the Lower Rhine.