Terrestrial invertebrates in central Amazonian floodplains must cope with annual long-term inundation. Parasites should be affected mainly indirectly through the specific life-cycles of their hosts. We studied the temporal structure of a beetle–fungus system at a central Amazonian blackwater floodplain (Rio Negro, Brazil). The host species Phaeoxantha aequinoctialis (Coleoptera, Carabidae, Cicindelinae) showed a seasonal, univoltine life-cycle triggered by the annual flood pulse. Infestation frequency of its fungal parasite, Laboulbenia phaeoxanthae (Ascomycota, Laboulbeniales), varied seasonally. However, the seasonality was opposed in host and parasite: the lowest infestation frequencies were observed during periods of highest beetle abundance and vice versa. Periods of lowest beetle abundance coincided with the end of the old generation, those with highest abundance with the appearance of new adults. The resulting annual patterns of a slow spread in the host population resembled the few records of temporal patterns from temperate regions. It is explicitly demonstrated that older adult female beetles are more frequently infested than younger ones. Future studies may reveal whether this is simply the result of specific host life-cycles (driven by a flood pulse, winter, or other factors), or might also be related to potentially easier infestation in older individuals.