After cataract extraction, bacterial endophthalmitis is most commonly caused by Staphylococcus epidermidis (70% of Endophthalmitis Vitrectomy Study isolates).1 This typically is seen as a moderately severe infection 5 to 7 days after surgery. Less commonly, 2 other forms of endophthalmitis can take place after cataract extraction. The first is a chronic, indolent endophthalmitis that manifests several months after surgery, usually caused by Propionibacterium acnes.12 A second, less common form of postoperative endophthalmitis is an early, fulminant type that usually manifests 2 to 4 days after surgery and is caused by streptococcal or staphylococcal species or by gram-negative organisms (most commonly Proteus mirabilis1). One reason we chose to study the intraocular penetration of moxifloxacin is that the MIC90 of moxifloxacin against the pathogens most commonly responsible for postoperative, posttraumatic, and bleb-associated endophthalmitis were generally lower than those for the other fluoroquinolone antibiotics we surveyed (Table 1). In our study, MIC90 values were far exceeded in the aqueous for a wide spectrum of pathogens in both the q2h and q6h groups, including Staphylococcus epidermidis, Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, Propionibacterium acnes, Haemophilus influenzae, Escherichia coli, Bacillus cereus, Neisseria gonorrhoeae, Proteus mirabilis, and other organisms. Concentration of moxifloxacin in the vitreous did not exceed the MIC90 for several organisms; however, in the q2h group, the MIC50 was exceeded for Staphylococcus epidermidis, Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Bacillus cereus, and other gram-negative pathogens.6 Moxifloxacin was unable to achieve intraocular levels effective against Pseudomonas species; furthermore, the MIC90 for Enterococcus species was exceeded only in the q2h aqueous group. Although Pseudomonas and Enterococcus species are very rarely encountered in postoperative endophthalmitis,1 0.5% moxifloxacin may not be a suitable treatment choice for intraocular infections known to be caused by these organisms.