The A(3)Sigma X--(u)3 Sigma(-)(g) electronic spectrum of the linear carbon chain radical HC7H has been recorded fully rotationally resolved. Cavity ring-down spectroscopy is used to record the origin band transition in direct absorption through a supersonically expanding planar plasma, discharging a diluted gas mixture of acetylene in helium. Rotational resolution is obtained by operating a commercial pulsed dye laser system in a second grating order configuration, resulting in a narrower bandwidth of about 0.035 cm (1). In total, 39 resolved P- and R-branch transitions are included in a standard fit, yielding for the first time accurate rotational constants for both electronic states. (C) 2010 Elsevier B.V. All rights reserved.