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 hydrogen, C1-C6alkyl, C1-C8haloalkyl or C3-C6cycloalkyl; R2 is cycloalkyl, cycloalkylalkyl, halocycloalkyl, cycloalkoxy, halocycloalkoxy, cycloalkylalkoxy, halocycloalkylalkoxy, alkoxyalkyl, cycloalkoxyalkoxyalkyl, haloalkoxyalkyl, trialkylsilyl, alkylthioalkyl, haloalkylthioalkyl, cycloalkylthio, halocycloalkylthio, cycloalkylalkylthio, halocycloalkylalkylthio, alkylsulfinylalkyl, alkylsulfonylalkyl, alkylsulfinyl, haloalkylsulfinyl, alkylsulfonyl, haloalkylsulfonyl, C2-C6alkenyl, C2-C6alkynyl, optionally substituted arylsulfonyl, optionally substituted aryloxy, optionally optionally substituted heteroaryloxy, optionally substituted arylthio, optionally substituted heteroarylthio; or R2 together with an adjacent carbon atom forms an optionally substituted fused ring; R3 is an optionally substituted aryl; R4 is hydrogen, halogen, C1-C6alkyl, C1-C6haloalkyl, C1-C8alkoxy, C1-C6haloalkoxy, hydroxy or cyano; and n is a whole number from 1 to 4; or an agrochemically usable salt form thereof.