The dye laser excitation spectrum of the vibronic band of HCF was observed between 18 190 and 18 422 cm−1 with Doppler-limited resolution. The observed spectrum exhibits a number of anomalies. The perturbations observed for Ka = 1 levels are well explained by an electronic Coriolis interaction with highly excited vibrational levels of the ground electronic state. Other anomalies are difficult to analyze, because most of them are local without showing any systematic frequency shifts. Some of the perturbed lines are found to have large Zeeman effects; thus the perturbations are ascribed to interactions with the lowest triplet state. The rotational analysis of the observed spectrum leads to the following molecular constants for HCF in the Ã1A(010) state: ν0 = 18 298.6927(71), B + C = 2.26710(34), B − C = 0.074(15), and the electronic Coriolis coupling term , all in cm−1 with standard errors in parentheses.