Lactococcus garvieae is an important pathogen of economic fish leading to high mortality and infection recurrence in summer or stress conditions. Biofilm formation is also an important mechanism of chronic infection and recurrence. However, the impact of biofilm and planktonic bacterial infection on fish immune response remains unclear. In this study, de novo sequencing was used to compare differences of the spleen transcriptome in planktonic- and biofilm-infected mullet.