Abstract. We consider a special class of solutions of the BKP hierarchy which we call τ -functions of hypergeometric type. These are series in Schur Q-functions over partitions, with coefficients parameterised by a function of one variable ξ , where the quantities ξ (k), k ∈ ޚ + , are integrals of motion of the BKP hierarchy. We show that this solution is, at the same time, a infinite soliton solution of a dual BKP hierarchy, where the variables ξ (k) are now related to BKP higher times. In particular, rational solutions of the BKP hierarchy are related to (finite) multi-soliton solution of the dual BKP hierarchy. The momenta of the solitons are given by the parts of partitions in the Schur Q-function expansion of the τ -function of hypergeometric type. We also show that the KdV and the NLS soliton τ -functions coinside the BKP τ -functions of hypergeometric type, evaluated at special point of BKP higher time; the variables ξ (which are BKP integrals of motions) being related to KdV and NLS higher times.2000 Mathematics Subject Classification. 35Q51, 35Q58, 05E05.1. Introduction. The BKP hierarchy was introduced in [1, 2] as a particular reduction of the KP hierarchy of integrable equations [1,7]. Like the well-known KP hierarchy, the BKP hierarchy possesses multi-soliton and rational solutions. In [3,4], the role of projective Schur functions (Q-functions) [6] in obtaining rational solutions of the BKP hierarchy was explained. In fact, the Q-functions are polynomial τ -function solutions of the BKP hierarchy Hirota equations and these are connected to the rational solutions through a change of dependent variables.In [9], certain hypergeometric series in Q-functions (see (85) below) were shown to be τ -functions of the BKP hierarchy. These τ -functions are series of the form