Secondary clines level down in the course of time if the gene flow is not interrupted. Temporally repeated sampling of populations in a cline allows the investigation not only of its occurrence but also of the estimation of the amount of ongoing gene flow. We reinvestigated an allozyme gradient in Carabus auronitens populations in the Westphalian Lowlands (northwestern Germany) 15 to 20 years after it had originally been recorded. A total of 977 individuals of this flightless woodland species from 29 sample sites were genotyped at the diallelic Est-1 locus in 2005-2006 and compared to former findings, collected in 1985-1994 from the same populations. Both data sets showed clinal variation. Pairwise differences between the samples of both data sets indicated significant decrease in the steepness of the cline during the past 15 to 20 years. The estimated average gene flow per generation is 0.6% of each beetle population. Ongoing gene flow in the flightless ground beetle C. auronitens led to a less pronounced cline despite a stable degree of fragmentation (and connectivity) of the landscape. Migration and gene flow were obviously enabled by the numerous hedgerows. The corridors are seen to be a prerequisite for migration between populations and for possible future range shifts of forest insect species.