A study was performed to establish the optimal concentration of traditional preservatives or fixatives such as formaldehyde and acidic Lugol's iodine, in order to preserve phytoplankton samples for long-term storage without the introduction of artifacts or other physical aberrations. The goal of the study was to avoid any visible morphological changes to the preserved cells, minimizing the errors induced by traditional preservative concentrations, and ensuring better accuracy of ecological analyses. We found that both formaldehyde and acidic Lugol's iodine have adverse effects on the preservation of samples. Trichodesmium erythraeum was found to be most susceptible to the effects of acidic Lugol's iodine, since it displayed the highest degree of chain fragmentation when this preservative was used. However, we found that 2.0% (v/v) formaldehyde, 2.5% (v/v) acidic Lugol's iodine, and 2.0% (v/v) formaldehyde+2.5%(v/v) acidic Lugol's iodine combined were most promising, with the latter the most effective even after 3 weeks of preservation. This study also revealed that, in general, the centric diatom species were more sensitive to long-term preservation than their pennate counterparts. The present study is significant as it sheds light on the damage endured by phytoplankton cells during long-term preservation, which can lead to erroneous and biased results upon analyses. The optimal concentration of preservative established experimentally from a wide variety of concentrations caused comparatively moderate changes to the cell dimensions as well as effectively prevented microbial contamination. © Copyright 2014, Mary Ann Liebert, Inc. 2014.