ABSTRACT: Myrionecta rubra is a cosmopolitan estuarine and neritic ciliate, known to cause ‘red-water’ blooms. The current study was conducted to achieve a better understanding of the relationship of photosynthetic performance and growth with feeding on cryptophyte algae in M. rubra. During the experiment the cryptophyte Teleaulax acuta was introduced for 2 consecutive growth periods (14 d each) and the cultures were then starved during 4 additional periods. In both high light (HL) and low light (LL) treatments, a significant decrease in per capita growth rates (μ) was observed over time (p < 0.05) in the absence of new prey. In the LL treatment, chlorophyll a content (chl a cell^–1), photosynthetic capacity (P_{max (cell)}), and photosynthetic efficiency (α_cell) increased after feeding and then declined during starvation. In the HL treatment, chl cell^–1 and α_cell also increased and then declined after feeding; however, P_{max (cell)} showed only a slight decrease with starvation. In both treatments, M. rubra appeared to undergo an acclimation-like response following declines in chl cell^–1, with increases in P_{max (chl)} and the light saturation parameter (I_k). While photosynthetic efficiency declined during starvation, overall photosynthetic capacity appeared to become uncoupled withgrowth over time. M. rubra demonstrated a high capacity for chl production (7 to 10 µg chl a ml^–1 d^–1) after feeding, which decreased during starvation. Declines in growth and photosynthetic parameters coincided with the loss of prey nuclei from M. rubra cells, implicating a possible functional role for retained prey nuclei. These data show that M. rubra can function phototrophically for extended periods without ingesting prey, but that feeding is periodically required for optimal growth and photosynthesis, especially in high light.