  We investigate Gromov-Witten invariants associated to exceptional classes for
primitive birational contractions on a Calabi-Yau threefold X. It was observed
in a previous paper that these invariants are locally defined, in that they can
be calculated from knowledge of an open neighbourhood of the exceptional locus
of the contraction; in this paper, we make this explicit.
  For Type I contractions (i.e. only finitely many exceptional curves), a
method is given for calculating the Gromov-Witten invariants, and these in turn
yield explicit expressions for the changes in the cubic form $D^3$ and the
linear form $D.c_2$ under the corresponding flop. For Type III contractions
(when a divisor E is contracted to a smooth curve C of singularities), there
are only two relevant Gromov-Witten numbers n(1) and n(2). Here n(2) is the
number (suitably defined) of simple pseudo-holomorphic rational curves
representing the class of a fibre of E over C, and n(1) the number of simple
curves representing half this class. Explicit formulae for n(1) and n(2) are
given (n(1) in terms of the singular fibres of E over C and n(2)=2g(C)-2). An
easy proof of these formulae is provided when g(C)>0. The main part of the
paper then gives a proof valid in general (including the case g(C)=0).
