In traditional multicarrier systems, the pilot carriers used to estimate the channel are placed as uniform as possible over the bandwidth. However, with the raise of cognitive radio systems, where the multicarrier system is used by the secondary users, parts of the bandwidth are not available for transmission as primary users are active in these bands. Therefore, the multicarrier system must introduce guard bands in which the carriers may not be used. Hence, a problem might occur when placing the pilot carriers. In this paper, we investigate the effect of the positions of the pilot carriers on the MSE performance of channel estimation, and look for the pilot carrier placement that minimizes the MSE. We do not restrict our attention to multicarrier systems with a cyclic prefix, but we also consider other types of guard interval that are used for multicarrier transmission. It is known that an equidistantly-spaced distribution of the pilot carriers is in general not the optimal placement of the pilot carriers, as the corresponding MSE can become very high. In this paper, we use a heuristic algorithm to search for the best pilot placement, which is able to deliver a pilot carrier placement that outperforms the maximum distance distribution from [S. Song and A. C. Singer, “Pilot-Aided OFDM Channel Estimation in the Presence of the Guard Band,” IEEE Trans. Commun., vol. 55, no. 8, pp. 1459-1465, Aug. 2008] in terms of the MSE, and results in an MSE that is close to the case where no guard bands are present. Based on the results of the algorithm, we can derive some simple rules of thumb to select the positions of the pilot carriers.