Benzonitrile (c-C6H5CN, where `c' indicates a cyclic structure), a polar proxy for benzene (c-C6H6), has the potential to serve as a highly convenient radio probe for aromatic chemistry, provided that this ring can be found in other astronomical sources beyond the molecule-rich prestellar cloud TMC-1. Here we present radio astronomical evidence of benzonitrile in four other prestellar, and possibly protostellar, sources: Serpens 1A, Serpens 1B, Serpens 2 and MC27/L1521F. These detections establish that benzonitrile is not unique to TMC-1; rather, aromatic chemistry appears to be widespread throughout the earliest stages of star formation, probably persisting at least until the initial formation of a protostar. The abundance of benzonitrile far exceeds predictions from models that well reproduce the abundances of carbon chains such as HC7N, a cyanpolyyne with the same heavy atoms, indicating that the chemistry responsible for planar carbon structures (as opposed to linear ones) in primordial sources is favourable but not well understood. The abundance of benzonitrile relative to carbon chain molecules displays sizable variations between sources within the Taurus and Serpens clouds, implying the importance of physical conditions and initial elemental reservoirs of the clouds themselves.