Context. HD 49434 is a bright (V = 5.75 m) and multiperiodic γ Dor-type pulsator that has been selected for the asteroseismic core programme of the CoRoT satellite. Aims: An extensive and detailed study has been carried out to investigate the pulsational content of this object on the basis of the 136.9 d (331291 useful datapoints) time series collected by the CoRoT satellite during the long-run LRa01 (October 2007-March 2008). Methods: The frequency analysis was carried out using the recently available software package SigSpec and Period04. Results: The results confirm HD 49434 as a complex pulsator with a very dense pulsation spectrum. A total of 1686 significant peaks are formally detected, essentially in the region below 15 d-1. No significant pulsation peaks are detected for frequencies higher than 30 d-1. Solar-type oscillations are not detected in this star. The reliability of such a high number of detected peaks has been investigated on the basis of two independent methods. As a result, we propose a total of 840 frequencies that can be assumed to be intrinsic to the star. Conclusions: The CoRoT mission provides very high-quality light curves and datasets, which are excellent for asteroseismic studies of complex pulsating stars beacause they allow investigation of the frequency content down to amplitude levels of a few μmag, unattainable with ground-based observations. In this way, hundreds of excited modes were detected in HD 49434. This is the first time that so many frequencies have been found in a γ Dor-type pulsator. The CoRoT space mission was developed and it is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programmes, Austria, Belgium, Brazil, Germany, and Spain.Tables 2, 3, 6-9, 11-14, 16-19, and 21-23 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/525/A23