The present invention relates to novel pyridazine derivatives of formula (I) as active ingredients which have microbiocidal activity, in particular fungicidal activity: wherein R1 is methyl, ethyl or isopropyl; R2 is chloro, fluoro, hydroxy or C1-C2alkoxy; R3 is H, chloro, fluoro, methoxy or C1-C3alkyl; R4 is chloro, fluoro or bromo; and R5 is H, fluoro or methoxy; or an agrochemically usable salt form thereof; with the proviso that when R1 is methyl, R2 is chloro and R3 is H, then R4 or R5 is different from fluoro.