We introduce cosymplectic circles and cosymplectic spheres, which are the analogues in the cosymplectic setting of contact circles and contact spheres. We provide a complete classification of compact 3-manifolds that admit a cosymplectic circle. The properties of tautness and roundness for a cosymplectic p-sphere are studied. To any taut cosymplectic circle on a three-dimensional manifold M we are able to canonically associate a complex structure and a conformal symplectic couple on M x R. We prove that a cosymplectic circle in dimension three is round if and only if it is taut. On the other hand, we provide examples in higher dimensions of cosymplectic circles which are taut but not round and examples of cosymplectic circles which are round but not taut. (C) 2015 Elsevier B.V. All rights reserved.