In this paper, performances of basic morphological filters are evaluated. Very simple output distribution expressions of erosions and openings are given for independent non-identically distributed inputs. The output means and variances for input signals plus white Gaussian, bi- exponential, and uniform noises are then analyzed and computed. These results are used to compare the performances of morphological filters with those of median filters, (alpha) - trimmed mean filters, ranked-order filters and running mean filters. The comparisons show that morphological filters achieve the best edge preservation for all the three kinds of noises. As regards to noise suppression, morphological filters are the best ones for uniform noise, median filters are optimal for bi-exponential noise, and running mean filters are optimal for Gaussian noise. Performances of (alpha) -trimmed mean filters spread between those of median and linear filters, while performances of ranked-order filters are compromises between those of erosions (or dilation) and median filters.