Femtosecond mid-infrared spectroscopy was applied to study the vibrational spectra and dynamics in the electronic excited states of semiconducting single-walled carbon nanotubes (SWNTs). The experiments were performed by exciting SWNTs dispersed individually in polymethylmethacrylate and polyvinyl alcohol polymer films with 40 fs laser pulses at 800 nm, and the resulting responses were monitored with broadband mid-infrared pulses ranging from 1510 to 1670 cm(-1). The structured spectra observed show vibrational bands with up-shifted frequencies by similar to 10-50 cm(-1) with respect to their ground-state counterparts. The observation provides direct evidence for the theoretically predicted lattice distortions in the electronic excited state. Analysis of the kinetics probed in the mid- and near-infrared regions provide an estimate of the time scales for the vibrational relaxation.