Mediterranean infralittoral soft bottoms are dominated by extensive seagrass meadows of the endemic phanerogam Posidonia oceanica (L.) Delile. Both leaves and rhizomes are substrates suitable for colonization by a vast array of epiphytic organisms, including foraminifera. Epiphytic foraminifera are very useful bioindicators, because of their relatively short life cycles and their quick reaction to environmental changes at global and local scales. To evaluate the environmental conditions reflected by the foraminiferal assemblages in P. oceanica-dominated environments, the FORAM Index (FI) has been slightly modified to include changes to the ecological categories defined in the original Langer A–D morphotype scheme. Sensitive, long-lived species are now represented by the SB and A* groups; the stress-tolerant taxa correspond to D*; and the small heterotrophic forms are still represented by B and C. Consequently, the FI was modified as follows: FI′ = 10 3 (PA*+PSB) + PD* + 2 × (PB+PC). The Posidonia oceanica foraminiferal assemblage contains very abundant B and C forms that remain more or less constant regardless of environmental conditions as they do not have as strict requirements for light and substrate as the A* and SB groups. Moreover, B and C taxa are not as well-adapted to stressful conditions as is D*. Therefore, to magnify the differences between, a priori, minimally altered and stressed areas, a new index is calculated. The “long vs. short life span” index (ILS), which is expressed as: ILS = (3.5 × (PA* + PSB) + 0.01)/(PD* + 0.01). Index ILS has a high correlation with the modified FORAM Index, which was developed to characterize water quality. Index ILS, can be used as a reference for the quality assessment and preservation of P. oceanica meadows.