Summary: Under a biaxial tension field, failure in a small region can give rise to simultaneous rapid crack extension along multiple paths. To gain insight into this phenomenon, the dynamic analysis of a cruciform crack growing from a point in a biaxial tension field is considered. An analytic solution is at present not available. However, by treating the cruciform arms as dislocation arrays, the problem is reduced to a pair of coupled integral equations. By following a scheme for plane crack analysis, these equations are uncoupled, and solved numerically by a standard technique for singular integral equations. Examination of the solution indicates that a plane crack is more likely to occur than a cruciform crack in a uniform biaxial tension field, and the crack openings will be larger.