We consider an analogue of the notion of instanton bundle on the projective 3-space, consisting of a class of rank-2 vector bundles defined on smooth Fano threefolds X of Picard number one, having even or odd determinant according to the parity of K X .We first construct a well-behaved irreducible component of their moduli spaces. Then, when the intermediate Jacobian of X is trivial, we look at the associated monads, hyperdeterminants and nets of quadrics. We also study one case where the intermediate Jacobian of X is non-trivial, namely when X is the intersection of two quadrics in 5 , relating instanton bundles on X to vector bundles of higher rank on a the curve of genus 2 naturally associated 2000 Mathematics Subject Classification. Primary 14J60, 14F05. Key words and phrases. Instanton bundles, monads, Fano threefolds, semiorthogonal decomposition, nets of quadrics.The author was partially supported by ANR-09-JCJC-0097-0 INTERLOW and ANR GEOLMI.1 * 3 (−r X ) ≃ 1 , * 2 (−r X ) ≃ 2 . Set U = Hom X ( 2 , 3 ), and note that U ≃ Hom X ( 1 , 2 ).