Heterocyclyl-substd. propanoic acids and their derivs., of formula (I), are new. R = formyl, tetrazolyl, nitrile, COOH or a gp. that can be hydrolysed to COOH; X = N or CR14; R2 = H, OH, NH2, alkylamino, dialkylamino, halo, alkyl, haloalkyl, alkoxy, haloalkoxy or alkylthio; R14 = H or 1-5 C alkyl; R3 = R2 or alkoxyimino; or CR3 + CR14 = 5 or 6 membered alkylene or alkenyl ring (opt. substd. by 1-2 alkyl) in which a methylene can be replaced by O, S, NH or alkylamino; R4, R5 = phenyl or naphthyl (both opt. substd. by one or more halo, NO2, CN, OH, alkyl, haloalkyl, alkoxy, haloalkyloxy, phenoxy, alkylthio, NH2, alkylamine or dialkylamino) or 3-7 C cycloalkyl; or the phenyl or naphthyl gps. are opt. bonded at the O- position (by a bond, CH2, O, S, SO2, ethylene, ethenylene or opt. alkylated amino); R6 = H, 1-8 C alkyl, 2-6 C alkenyl, 3-6 C alkynyl, 3-8 C cycloalkyl (all opt. substd. by 1 or more halo, NO2, CN, alkoxy 3-6 C alkenyloxy, 3-6 C alkynyloxy, alkylthio, haloalkoxy, alkylcarbonyl, alkoxycarbonyl, 3-8 C alkylcarbonylalkyl, alkylamino, dialkylamino, phenyl, Ph or OPh), Ar or Het; Y = S, O or bond; Q = S, O, SO, SO2 or bond; Ph = substd. phenyl, where substits. are halo, NO2, CN, alkyl, haloalkyl, alkoxy, haloalkoxy or alkylthio; Ar = phenyl or naphthyl (both opt. substd. by 1 or more halo, NO2, CN, OH, NH2, alkylamino, dialkylamino or OCH2CH2O); Het = 5 or 6 membered heteroaromatic with 1-3 N atoms and/or one S or O (opt. substd. by 1-4 halo and/or 1 or 2 alkyl, haloalkyl, alkoxy, haloalkoxy, alkylthio, phenyl, phenoxy or phenyl carbonyl (where the phenyl is opt. substd. by 1-5 halo and/or 1-3 alkyl, haloalkyl, alkoxy, haloalkoxy and/or alkylthio)); alkyl and alkoxy have 1-4 C unless otherwise stated; provided that R6 is not H when Q = bond.