To assess the involvement of the genus Variovorax and the linuron hydrolase gene libA in in situ linuron degradation in agricultural fields, changes in Variovorax community size and composition, in libA abundance and in linuron mineralization capacity were monitored in field soil plots either treated or not with a linuron-containing herbicide mixture. Changes in Variovorax community composition and increases in libA gene copy numbers corresponded to increases in linuron mineralization capacity in the plot treated with the herbicide mixture, suggesting that Variovorax and libA proliferated as a response to linuron in the field and their contribution to in situ linuron degradation. The response of the Variovorax community composition was especially due to the proliferation of a Variovorax phylotype (phylotype D) that differed from Variovorax phylotypes previously associated with linuron degradation and libA abundance in the same soil, suggesting the occurrence of horizontal gene transfer of libA in the examined field. The involvement of Variovorax phylotype D and libA in linuron degradation in the examined soil was supported by lab soil microcosm experiments that showed further increase or decrease of both the population size of phylotype D and libA numbers when linuron was added or omitted, respectively. Attempts to enrich in suspended cultures and isolate the corresponding organism from the soil were unsuccessful. In those enrichments, Variovorax phylotype D was replaced by other Variovorax phylotypes showing that linuron-degrading strains isolated by liquid enrichment cultures are not always representatives of those responsive to linuron in the field, although the genus specificity of linuron degradation was retained.