In laying hen facilities drinking water additives are used to support the hens natural resistance. It seems that these productshave a side-effect on the life cycle of the ecto-parasitic miteDermanyssus gallinae; either because mites do not approach the laying hens by the products repellent effects or by making the blood indigestible for the poultry red mite. Whether these products do need a registration as veterinary medicine or as biocide was not the subject of this study. The aim ofthis studywas to test the effect ofHensupp+ and Alphamites DWon D. gallinaefemale adults’ability to lay eggs and on the D. gallinaenymphs’ability to moult. Here we measuredindirect the effect of the products onthe lifecycle of D. gallinae. D. gallinaemites were collected at 3 commercial farms using Alphamites DW and at 4 commercialfarms using Hensupp+. Mites were also collected at a commercial control farm where no water additives were used, but only a silica product to control D. gallinae. Recently engorged female adults and nymphs were individually placed in 96 well plates enabling to determine mite egg production or moulting. No significant differences were found in the percentage of adult mites producing eggs or the percentage of moulting nymphs of the collected mites from the farms with drinking water additives, compared to the mites from the control farm. A trend however was seen for a lower percentage of moulting nymphs from farms which supplyAlphamites DW compared to the percentage of moultingnymphs from the control farm.This observational studywas carried out at seven commercial farms of which it can be expected that they carried outthe advised treatment protocol, but of which we are not sure. Neither do we know if there werefactors present (such as health status of the flock and water and feed quality)which could have influenced the test results. In this test, we determined a small part of the possible effects of the two drinking water additives on red mites and therefore we are not able to draw conclusions about the total effect of the two water additives on the poultry red mite.