microRNAs (miRNAs) are critical to heart development and disease. Emerging research indicates that regulated precursor processing can give rise to an unexpected diversity of miRNA variants. We subjected small RNA from murine HL-1 cardiomyocyte cells to next generation sequencing to investigate the relevance of such diversity to cardiac biology. ∼40 million tags were mapped to known miRNA hairpin sequences as deposited in miRBase version 16, calling 403 generic miRNAs as appreciably expressed. Hairpin arm bias broadly agreed with miRBase annotation, although 44 miR* were unexpectedly abundant (>20% of tags); conversely, 33 -5p/-3p annotated hairpins were asymmetrically expressed. Overall, variability was infrequent at the 5' start but common at the 3' end of miRNAs (5.2% and 52.3% of tags, respectively). Nevertheless, 105 miRNAs showed marked 5' isomiR expression (>20% of tags). Among these was miR-133a, a miRNA with important cardiac functions, and we demonstrated differential mRNA targeting by two of its prevalent 5' isomiRs. Analyses of miRNA termini and base-pairing patterns around Drosha and Dicer cleavage regions confirmed the known bias towards uridine at the 5' most position of miRNAs, as well as supporting the thermodynamic asymmetry rule for miRNA strand selection and a role for local structural distortions in fine tuning miRNA processing. We further recorded appreciable expression of 5 novel miR*, 38 extreme variants and 8 antisense miRNAs. Analysis of genome-mapped tags revealed 147 novel candidate miRNAs. In summary, we revealed pronounced sequence diversity among cardiomyocyte miRNAs, knowledge of which will underpin future research into the mechanisms involved in miRNA biogenesis and, importantly, cardiac function, disease and therapy.