The spatial pattern of flea (Siphonaptera: Pulicidae) infestation risk in cats and dogs across Great Britain is quantified, using data collected from a national survey undertaken in 2018, with particular attention given to the association between insecticidal treatment and infestation risk. Flea infestation risk declined significantly from south to north. None of the factors: pet breed, sex, neutered status, or whether the pet had been abroad, showed any relationship with the underlying geographic distribution, which is most likely to be associated with climatic factors. However, overall, only 23.6% of the cats and 35% of the dogs inspected had been treated with identifiable flea products that were still ‘in date’ at the point of inspection. The percentage of owners treating their pet broadly followed infestation risk. The insecticide fipronil is a common active in a wide range of flea treatments and was the most frequently applied insecticide class, particularly in cats. However, 62% of cats and 45% of dogs that had been treated with a fipronil-based product that was ‘in date’ at the point of inspection still had fleas. Persistent flea infestation is likely to be due to a range of factors, including compliance and application failure, but the data provide strong inferential evidence for a lack of efficacy of fipronil-based products. Given the ubiquity of flea infestation, this finding and the relatively low-level of treatment compliance, highlight a clear need for greater owner education about the importance of flea management and a better understanding of the efficacy of different products.